btrfs seed question
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Is there maybe an ioctl or something else that might trigger a similar action?
Thanks,
-Joseph
Quoted 11 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?
Thanks, Anand
Quoted 7 lines
Thanks,
-Joseph
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Quoted 10 lines · On Thu, 12 Oct 2017 12:18:01 +0800 Anand Jain <[email protected]> wrote:
Quoted 9 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Quoted 7 lines
Quoted 2 lines
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?
Not quite. While the seed device is still connected I would like to force some files over to the rw device. The use case is basically a much slower link to a seed device holding significantly more data than we currently need. An example would be a slower iscsi link to the seed device and a local rw ssd. I would like fast access to a certain subset of files, likely larger than the memory cache will accommodate. If at a later time I want to discard the image as a whole I could unmount the file system or if I want a full local copy I could delete the seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
I'm not necessarily looking for a new feature addition, just if there is some existing call that I can make to push specific files from the slow mirror to the fast one. If I had to push a significant amount of metadata that would be fine, but the file contents feeding some computations might be large and useful only to certain clients.
So far I found that I can re-write the file with the same contents and thanks to the lack of online dedupe these writes land on the rw mirror so later reads to that file should not hit the slower mirror. By the way, if I'm misunderstanding how the read process would work after the file push please correct me.
I hope this makes sense but I'll try to clarify further if you have more questions.
-Joseph
Quoted 11 lines
Thanks, Anand
Quoted 7 lines
Thanks,
-Joseph
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Quoted 22 lines · Joseph Dunn
Quoted 10 lines · On Thu, 12 Oct 2017 12:18:01 +0800 Anand Jain <[email protected]> wrote:
Quoted 9 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Quoted 7 lines
Quoted 2 lines
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?
Quoted 12 lines
Not quite. While the seed device is still connected I would like to force some files over to the rw device. The use case is basically a much slower link to a seed device holding significantly more data than we currently need. An example would be a slower iscsi link to the seed device and a local rw ssd. I would like fast access to a certain subset of files, likely larger than the memory cache will accommodate. If at a later time I want to discard the image as a whole I could unmount the file system or if I want a full local copy I could delete the seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
Multiple seeds?
Seed A has everything, is remote. Create sprout B also remotely, deleting the things you don't absolutely need, then make it a seed. Now via iSCSI you can mount both A and B seeds. Add local rw sprout C to seed B, then delete B to move files to fast local storage.
Signature · 1 line
Chris Murphy
Quoted 37 lines · On Thu, 12 Oct 2017 15:32:24 +0100 Chris Murphy <[email protected]> wrote:
Quoted 22 lines · Joseph Dunn
Quoted 10 lines · On Thu, 12 Oct 2017 12:18:01 +0800 Anand Jain <[email protected]> wrote:
Quoted 9 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Quoted 7 lines
Quoted 2 lines
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?Quoted 12 lines
Not quite. While the seed device is still connected I would like to force some files over to the rw device. The use case is basically a much slower link to a seed device holding significantly more data than we currently need. An example would be a slower iscsi link to the seed device and a local rw ssd. I would like fast access to a certain subset of files, likely larger than the memory cache will accommodate. If at a later time I want to discard the image as a whole I could unmount the file system or if I want a full local copy I could delete the seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
Quoted 7 lines
Multiple seeds?
Seed A has everything, is remote. Create sprout B also remotely, deleting the things you don't absolutely need, then make it a seed. Now via iSCSI you can mount both A and B seeds. Add local rw sprout C to seed B, then delete B to move files to fast local storage.
Interesting thought. I haven't tried working with multiple seeds but I'll see what that can do. I will say that this approach would require more pre-planning meaning that the choice of fast files could not be made based on current access patterns to tasks at hand. This might make sense for a core set of files, but it doesn't quite solve the whole problem.
-Joseph
Quoted 4 lines
Signature · 2 lines
-- Chris Murphy
Quoted 47 lines · Joseph Dunn
Quoted 37 lines · On Thu, 12 Oct 2017 15:32:24 +0100 Chris Murphy <[email protected]> wrote:
Quoted 22 lines · Joseph Dunn
Quoted 10 lines · On Thu, 12 Oct 2017 12:18:01 +0800 Anand Jain <[email protected]> wrote:
Quoted 9 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Quoted 7 lines
Quoted 2 lines
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?Quoted 12 lines
Not quite. While the seed device is still connected I would like to force some files over to the rw device. The use case is basically a much slower link to a seed device holding significantly more data than we currently need. An example would be a slower iscsi link to the seed device and a local rw ssd. I would like fast access to a certain subset of files, likely larger than the memory cache will accommodate. If at a later time I want to discard the image as a whole I could unmount the file system or if I want a full local copy I could delete the seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
Quoted 7 lines
Multiple seeds?
Seed A has everything, is remote. Create sprout B also remotely, deleting the things you don't absolutely need, then make it a seed. Now via iSCSI you can mount both A and B seeds. Add local rw sprout C to seed B, then delete B to move files to fast local storage.
Quoted 6 lines
Interesting thought. I haven't tried working with multiple seeds but I'll see what that can do. I will say that this approach would require more pre-planning meaning that the choice of fast files could not be made based on current access patterns to tasks at hand. This might make sense for a core set of files, but it doesn't quite solve the whole problem.
I think the use case really dictates a dynamic solution that's smarter than either of the proposed ideas (mine or yours). Basically you want something that recognizes slow vs fast storage, and intelligently populates fast storage with frequently used files.
Ostensibly this is the realm of dmcache. But I can't tell you whether dmcache or via LVM tools, if it's possible to set the proper policy to make it work for your use case. And also I have no idea how to set it up after the fact, on an already created file system, rather than block devices.
The hot vs cold files thing, is something I thought the VFS folks were looking into.
Signature · 1 line
Chris Murphy
Quoted 61 lines · On Thu, 12 Oct 2017 16:30:36 +0100 Chris Murphy <[email protected]> wrote:
Quoted 47 lines · Joseph Dunn
Quoted 37 lines · On Thu, 12 Oct 2017 15:32:24 +0100 Chris Murphy <[email protected]> wrote:
Quoted 22 lines · Joseph Dunn
Quoted 10 lines · On Thu, 12 Oct 2017 12:18:01 +0800 Anand Jain <[email protected]> wrote:
Quoted 9 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Quoted 7 lines
Quoted 2 lines
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?Quoted 12 lines
Not quite. While the seed device is still connected I would like to force some files over to the rw device. The use case is basically a much slower link to a seed device holding significantly more data than we currently need. An example would be a slower iscsi link to the seed device and a local rw ssd. I would like fast access to a certain subset of files, likely larger than the memory cache will accommodate. If at a later time I want to discard the image as a whole I could unmount the file system or if I want a full local copy I could delete the seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
Quoted 7 lines
Multiple seeds?
Seed A has everything, is remote. Create sprout B also remotely, deleting the things you don't absolutely need, then make it a seed. Now via iSCSI you can mount both A and B seeds. Add local rw sprout C to seed B, then delete B to move files to fast local storage.
Quoted 6 lines
Interesting thought. I haven't tried working with multiple seeds but I'll see what that can do. I will say that this approach would require more pre-planning meaning that the choice of fast files could not be made based on current access patterns to tasks at hand. This might make sense for a core set of files, but it doesn't quite solve the whole problem.
I think the use case really dictates a dynamic solution that's smarter than either of the proposed ideas (mine or yours). Basically you want something that recognizes slow vs fast storage, and intelligently populates fast storage with frequently used files.
Quoted 9 lines
Ostensibly this is the realm of dmcache. But I can't tell you whether dmcache or via LVM tools, if it's possible to set the proper policy to make it work for your use case. And also I have no idea how to set it up after the fact, on an already created file system, rather than block devices.
The hot vs cold files thing, is something I thought the VFS folks were looking into.
As a consumer of the file system data I tend to see things at a file level rather than as blocks, but from a block level this does feel dmcache-ish and I'll look into it.
I did try the multiple seeds approach and correct me if I'm wrong, but once the files are deleted in the second seed they are no longer accessible in anything sprouted from that. The fully dynamic solution would be nice, but I'm perfectly happy to pick the files for the ssd at run time, just not at file system preparation time. In any case, I may fall back on inotify and overwriting file contents if I don't end up with a better solution using dmcache or LVM tricks.
-Joseph Dunn
Am Thu, 12 Oct 2017 11:50:20 -0400
schrieb Joseph Dunn <[email protected]>:
Quoted 41 lines
Quoted 5 lines · On Thu, 12 Oct 2017 16:30:36 +0100 Chris Murphy <[email protected]> wrote:
Quoted 3 lines · Joseph Dunn
On Thu, 12 Oct 2017 15:32:24 +0100
Chris Murphy <[email protected]> wrote:[...]
[...]
[...]
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[...]Quoted 13 lines
Quoted 6 lines
Interesting thought. I haven't tried working with multiple seeds but I'll see what that can do. I will say that this approach would require more pre-planning meaning that the choice of fast files could not be made based on current access patterns to tasks at hand. This might make sense for a core set of files, but it doesn't quite solve the whole problem.
I think the use case really dictates a dynamic solution that's smarter than either of the proposed ideas (mine or yours). Basically you want something that recognizes slow vs fast storage, and intelligently populates fast storage with frequently used files.
Quoted 9 lines
Ostensibly this is the realm of dmcache. But I can't tell you whether dmcache or via LVM tools, if it's possible to set the proper policy to make it work for your use case. And also I have no idea how to set it up after the fact, on an already created file system, rather than block devices.
The hot vs cold files thing, is something I thought the VFS folks were looking into.
As a consumer of the file system data I tend to see things at a file level rather than as blocks, but from a block level this does feel dmcache-ish and I'll look into it.
Quoted 7 lines
I did try the multiple seeds approach and correct me if I'm wrong, but once the files are deleted in the second seed they are no longer accessible in anything sprouted from that. The fully dynamic solution would be nice, but I'm perfectly happy to pick the files for the ssd at run time, just not at file system preparation time. In any case, I may fall back on inotify and overwriting file contents if I don't end up with a better solution using dmcache or LVM tricks.
Would "btrfs filesystem defrag" not rewrite files to local storage?
Using inotify, you could work with two queues "defrag" and "done". Add files via inotify to the defrag queue unless it's already in one of the queues. Then move it to the done queue while running defrag on each files of the other queue.
You can then use the done queue as a seed for defrag queue on newly provisioned systems.
Signature · 4 lines
Regards, Kai Replies to list-only preferred.
Quoted 61 lines · Chris Murphy
Quoted 47 lines · Joseph Dunn
Quoted 37 lines · On Thu, 12 Oct 2017 15:32:24 +0100 Chris Murphy <[email protected]> wrote:
Quoted 22 lines · Joseph Dunn
Quoted 10 lines · On Thu, 12 Oct 2017 12:18:01 +0800 Anand Jain <[email protected]> wrote:
Quoted 9 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Quoted 7 lines
Quoted 2 lines
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?Quoted 12 lines
Not quite. While the seed device is still connected I would like to force some files over to the rw device. The use case is basically a much slower link to a seed device holding significantly more data than we currently need. An example would be a slower iscsi link to the seed device and a local rw ssd. I would like fast access to a certain subset of files, likely larger than the memory cache will accommodate. If at a later time I want to discard the image as a whole I could unmount the file system or if I want a full local copy I could delete the seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
Quoted 7 lines
Multiple seeds?
Seed A has everything, is remote. Create sprout B also remotely, deleting the things you don't absolutely need, then make it a seed. Now via iSCSI you can mount both A and B seeds. Add local rw sprout C to seed B, then delete B to move files to fast local storage.
Quoted 6 lines
Interesting thought. I haven't tried working with multiple seeds but I'll see what that can do. I will say that this approach would require more pre-planning meaning that the choice of fast files could not be made based on current access patterns to tasks at hand. This might make sense for a core set of files, but it doesn't quite solve the whole problem.
I think the use case really dictates a dynamic solution that's smarter than either of the proposed ideas (mine or yours). Basically you want something that recognizes slow vs fast storage, and intelligently populates fast storage with frequently used files.
Quoted 5 lines
Ostensibly this is the realm of dmcache. But I can't tell you whether dmcache or via LVM tools, if it's possible to set the proper policy to make it work for your use case. And also I have no idea how to set it up after the fact, on an already created file system, rather than block devices.
It is possible with dm-cache, but not at the file level (as befits any good block layer, it doesn't care about files, just blocks). Despite that, it should work reasonably fine (I've done this before with NBD and ATAoE devices, and it worked perfectly, so I would expect it to work just as well for iSCSI), and actually may do better than caching whole files locally depending on your workload.
As far as setup after the fact, the degree of difficulty depends on whether or not you want to use LVM. Without LVM, you should have no issue just setting up a device-mapper table for caching, you just need enough room on the local SSD for both the cache data and cache metadata partition. When you create the table using dmsetup 9and eventually in /etc/dmtab), it won't wipe the filesystem on the origin device, so you can easily add a cache to anything this way, even if it's an existing filesystem (though you will need to unmount the filesystem to add the cache). All things considered, it's no worse than setting it up on a brand new device, the hardest part is making sure you get the device sizes right for the device mapper table.
With LVM however, it's somewhat more complicated, because it refuses to work with anything it's not managing already, so you would have to reprovision the iSCSI device as a PV, add it to the local VG, and then work with that.
If you don't mind reprovisioning, I would actually suggest bcache instead of LVM here though. It's less complicated to add and remove caches, does somewhat better in recent versions of intelligently deciding what to cache locally, and is also significantly less likely to slow down the rest of your system than LVM (any management operations will have to hit that remote iSCSI device, and LVM does a lot more with data on the disk than bcache does).
Quoted 3 lines
The hot vs cold files thing, is something I thought the VFS folks were looking into.
I was under that impression too, but I haven't seen anything relating to it recently (though I'm not subscribed to the linux-vfs list, so there may be discussion there I'm not seeing).
Quoted 9 lines
Quoted 9 lines
Quoted 9 lines
Not quite. While the seed device is still connected I would like to
force some files over to the rw device. The use case is basically a
much slower link to a seed device holding significantly more data than
we currently need. An example would be a slower iscsi link to the seed
device and a local rw ssd. I would like fast access to a certain subset
of files, likely larger than the memory cache will accommodate. >>> If at a later time I want to discard the image as a whole I could unmount the file system or
Interesting. If you had not brought the idea of using seed device, I would have still thinking about bcache device as a solution for the above use case.
However bcache device won't help to solve the further use case requirements as below..
Quoted 5 lines
Quoted 5 lines
Quoted 5 lines
if I want a full local copy I could delete the
seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
I am thinking why not indeed leverage seed/sprout for this. Let me see how I (or if anyone could) allocate time to experiment on this.
Thanks for your use case !
-Anand
Am Thu, 12 Oct 2017 09:20:28 -0400
schrieb Joseph Dunn <[email protected]>:
Quoted 40 lines
Quoted 10 lines · On Thu, 12 Oct 2017 12:18:01 +0800 Anand Jain <[email protected]> wrote:
Quoted 9 lines · Joseph Dunn
After seeing how btrfs seeds work I wondered if it was possible to push specific files from the seed to the rw device. I know that removing the seed device will flush all the contents over to the rw device, but what about flushing individual files on demand?
I found that opening a file, reading the contents, seeking back to 0, and writing out the contents does what I want, but I was hoping for a bit less of a hack.
Quoted 7 lines
Quoted 2 lines
Is there maybe an ioctl or something else that might trigger a similar action?
You mean to say - seed-device delete to trigger copy of only the
specified or the modified files only, instead of whole of seed-device ? What's the use case around this ?Not quite. While the seed device is still connected I would like to force some files over to the rw device. The use case is basically a much slower link to a seed device holding significantly more data than we currently need. An example would be a slower iscsi link to the seed device and a local rw ssd. I would like fast access to a certain subset of files, likely larger than the memory cache will accommodate. If at a later time I want to discard the image as a whole I could unmount the file system or if I want a full local copy I could delete the seed-device to sync the fs. In the mean time I would have access to all the files, with some slower (iscsi) and some faster (ssd) and the ability to pick which ones are in the faster group at the cost of one content transfer.
I'm not necessarily looking for a new feature addition, just if there is some existing call that I can make to push specific files from the slow mirror to the fast one. If I had to push a significant amount of metadata that would be fine, but the file contents feeding some computations might be large and useful only to certain clients.
Quoted 8 lines
So far I found that I can re-write the file with the same contents and thanks to the lack of online dedupe these writes land on the rw mirror so later reads to that file should not hit the slower mirror. By the way, if I'm misunderstanding how the read process would work after the file push please correct me.
I hope this makes sense but I'll try to clarify further if you have more questions.
You could try to wrap something like bcache ontop of the iscsi device, then make it a read-mostly cache (like bcache write-around mode). This probably involves rewriting the iscsi contents to add a bcache header. You could try mdcache instead.
Then you sacrifice a few gigabytes of local SSD storage of the caching layer.
I guess that you're sharing the seed device with different machines. As bcache will add a protective superblock, you may need to thin-clone the seed image on the source to have independent superblocks per each bcache instance. Not sure how this applies to mdcache as I never used it.
But the caching approach is probably the easiest way to go for you. And it's mostly automatic once deployed: you don't have to manually choose which files to move to the sprout...
Signature · 4 lines
Regards, Kai Replies to list-only preferred.