Regarding ext4 extent allocation strategy

Our team in Microsoft, which works on the Linux SMB3 client kernel filesystem has recently been exploring the use of fscache on top of ext4 for caching the network filesystem data for some customer workloads.

However, the maintainer of fscache (David Howells) recently warned us that a few other extent based filesystem developers pointed out a theoretical bug in the current implementation of fscache/cachefiles. It currently does not maintain a separate metadata for the cached data it holds, but instead uses the sparseness of the underlying filesystem to track the ranges of the data that is being cached.
The bug that has been pointed out with this is that the underlying filesystems could bridge holes between data ranges with zeroes or punch hole in data ranges that contain zeroes. (@David please add if I missed something).

David has already begun working on the fix to this by maintaining the metadata of the cached ranges in fscache itself.
However, since it could take some time for this fix to be approved and then backported by various distros, I'd like to understand if there is a potential problem in using fscache on top of ext4 without the fix. If ext4 doesn't do any such optimizations on the data ranges, or has a way to disable such optimizations, I think we'll be okay to use the older versions of fscache even without the fix mentioned above.

Quoted 17 lines · Shyam Prasad N

Our team in Microsoft, which works on the Linux SMB3 client kernel filesystem has recently been exploring the use of fscache on top of ext4 for caching the network filesystem data for some customer workloads.

However, the maintainer of fscache (David Howells) recently warned us that a few other extent based filesystem developers pointed out a theoretical bug in the current implementation of fscache/cachefiles. It currently does not maintain a separate metadata for the cached data it holds, but instead uses the sparseness of the underlying filesystem to track the ranges of the data that is being cached.
The bug that has been pointed out with this is that the underlying filesystems could bridge holes between data ranges with zeroes or punch hole in data ranges that contain zeroes. (@David please add if I missed something).

Quoted 8 lines

David has already begun working on the fix to this by maintaining the metadata of the cached ranges in fscache itself.
However, since it could take some time for this fix to be approved and then backported by various distros, I'd like to understand if there is a potential problem in using fscache on top of ext4 without the fix. If ext4 doesn't do any such optimizations on the data ranges, or has a way to disable such optimizations, I think we'll be okay to use the older versions of fscache even without the fix mentioned above.

Yes, the tuning knob you are looking for is:

What:           /sys/fs/ext4/<disk>/extent_max_zeroout_kb
Date:           August 2012
Contact:        "Theodore Ts'o" <[email protected]>

Description:
The maximum number of kilobytes which will be zeroed out in preference to creating a new uninitialized extent when manipulating an inode's extent tree. Note that using a larger value will increase the variability of time necessary to complete a random write operation (since a 4k random write might turn into a much larger write due to the zeroout operation).

(From Documentation/ABI/testing/sysfs-fs-ext4)

The basic idea here is that with a random workload, with HDD's, the cost of writing a 16k random write is not much more than the time to write a 4k random write; that is, the cost of HDD seeks dominates. There is also a cost in having a many additional entries in the extent tree. So if we have a fallocated region, e.g:

    +-------------+---+---+---+----------+---+---+---------+
... + Uninit (U)  | W | U | W |   Uninit | W | U | Written | ...
    +-------------+---+---+---+----------+---+---+---------+

It's more efficient to have the extent tree look like this

    +-------------+-----------+----------+---+---+---------+
... + Uninit (U)  |  Written  |   Uninit | W | U | Written | ...
    +-------------+-----------+----------+---+---+---------+

And just simply write zeros to the first "U" in the above figure.

The default value of extent_max_zeroout_kb is 32k. This optimization can be disabled by setting extent_max_zeroout_kb to 0. The downside of this is a potential degredation of a random write workload (using for example the fio benchmark program) on that file system.

Cheers,

- Ted

Also, is this parameter also respected when a hole is punched in the middle of an allocated data extent? i.e. is there still a possibility that a punched hole does not translate to splitting the data extent, even when extent_max_zeroout_kb is set to 0?

Quoted 5 lines · Shyam Prasad N

Also, is this parameter also respected when a hole is punched in the middle of an allocated data extent? i.e. is there still a possibility that a punched hole does not translate to splitting the data extent, even when extent_max_zeroout_kb is set to 0?

Ext4 doesn't ever try to zero blocks as part of a punch operation. It's true a file system is allowed to do it, but I would guess most wouldn't, since the presumption is that userspace is actually trying to free up disk space, and so you would want to release the disk blocks in the punch hole case.

The more interesting one is the FALLOC_FL_ZERO_RANGE_FL operation, which *should* work by transitioning the extent to be uninitialized, but there might be cases where writing a few zero blocks might be faster in some cases. That should use the same code path which resepects the max_zeroout configuration parameter for ext4.

Cheers,

- Ted

Quoted 17 lines · Shyam Prasad N

Our team in Microsoft, which works on the Linux SMB3 client kernel filesystem has recently been exploring the use of fscache on top of ext4 for caching the network filesystem data for some customer workloads.

However, the maintainer of fscache (David Howells) recently warned us that a few other extent based filesystem developers pointed out a theoretical bug in the current implementation of fscache/cachefiles. It currently does not maintain a separate metadata for the cached data it holds, but instead uses the sparseness of the underlying filesystem to track the ranges of the data that is being cached.
The bug that has been pointed out with this is that the underlying filesystems could bridge holes between data ranges with zeroes or punch hole in data ranges that contain zeroes. (@David please add if I missed something).

Quoted 8 lines

David has already begun working on the fix to this by maintaining the metadata of the cached ranges in fscache itself.
However, since it could take some time for this fix to be approved and then backported by various distros, I'd like to understand if there is a potential problem in using fscache on top of ext4 without the fix. If ext4 doesn't do any such optimizations on the data ranges, or has a way to disable such optimizations, I think we'll be okay to use the older versions of fscache even without the fix mentioned above.

Yes, the tuning knob you are looking for is:

What: /sys/fs/ext4/<disk>/extent_max_zeroout_kb
Date: August 2012
Contact: "Theodore Ts'o" <[email protected]>
Description:
The maximum number of kilobytes which will be zeroed out in preference to creating a new uninitialized extent when manipulating an inode's extent tree. Note that using a larger value will increase the variability of time necessary to complete a random write operation (since a 4k random write might turn into a much larger write due to the zeroout operation).

(From Documentation/ABI/testing/sysfs-fs-ext4)

The basic idea here is that with a random workload, with HDD's, the cost of writing a 16k random write is not much more than the time to write a 4k random write; that is, the cost of HDD seeks dominates. There is also a cost in having a many additional entries in the extent tree. So if we have a fallocated region, e.g:

    +-------------+---+---+---+----------+---+---+---------+
... + Uninit (U)  | W | U | W |   Uninit | W | U | Written | ...
    +-------------+---+---+---+----------+---+---+---------+

It's more efficient to have the extent tree look like this

    +-------------+-----------+----------+---+---+---------+
... + Uninit (U)  |  Written  |   Uninit | W | U | Written | ...
    +-------------+-----------+----------+---+---+---------+

And just simply write zeros to the first "U" in the above figure.

The default value of extent_max_zeroout_kb is 32k. This optimization can be disabled by setting extent_max_zeroout_kb to 0. The downside of this is a potential degredation of a random write workload (using for example the fio benchmark program) on that file system.

Cheers,

- Ted

Hi Ted and David,

Quoted 60 lines · Theodore Y. Ts'o

Quoted 17 lines · Shyam Prasad N

Our team in Microsoft, which works on the Linux SMB3 client kernel filesystem has recently been exploring the use of fscache on top of ext4 for caching the network filesystem data for some customer workloads.

However, the maintainer of fscache (David Howells) recently warned us that a few other extent based filesystem developers pointed out a theoretical bug in the current implementation of fscache/cachefiles. It currently does not maintain a separate metadata for the cached data it holds, but instead uses the sparseness of the underlying filesystem to track the ranges of the data that is being cached.
The bug that has been pointed out with this is that the underlying filesystems could bridge holes between data ranges with zeroes or punch hole in data ranges that contain zeroes. (@David please add if I missed something).

Quoted 8 lines

David has already begun working on the fix to this by maintaining the metadata of the cached ranges in fscache itself.
However, since it could take some time for this fix to be approved and then backported by various distros, I'd like to understand if there is a potential problem in using fscache on top of ext4 without the fix. If ext4 doesn't do any such optimizations on the data ranges, or has a way to disable such optimizations, I think we'll be okay to use the older versions of fscache even without the fix mentioned above.

Yes, the tuning knob you are looking for is:

What: /sys/fs/ext4/<disk>/extent_max_zeroout_kb
Date: August 2012
Contact: "Theodore Ts'o" <[email protected]>
Description:
The maximum number of kilobytes which will be zeroed out in preference to creating a new uninitialized extent when manipulating an inode's extent tree. Note that using a larger value will increase the variability of time necessary to complete a random write operation (since a 4k random write might turn into a much larger write due to the zeroout operation).

(From Documentation/ABI/testing/sysfs-fs-ext4)

The basic idea here is that with a random workload, with HDD's, the cost of writing a 16k random write is not much more than the time to write a 4k random write; that is, the cost of HDD seeks dominates. There is also a cost in having a many additional entries in the extent tree. So if we have a fallocated region, e.g:

    +-------------+---+---+---+----------+---+---+---------+
... + Uninit (U)  | W | U | W |   Uninit | W | U | Written | ...
    +-------------+---+---+---+----------+---+---+---------+

It's more efficient to have the extent tree look like this

    +-------------+-----------+----------+---+---+---------+
... + Uninit (U)  |  Written  |   Uninit | W | U | Written | ...
    +-------------+-----------+----------+---+---+---------+

Quoted 7 lines

And just simply write zeros to the first "U" in the above figure.

The default value of extent_max_zeroout_kb is 32k. This optimization can be disabled by setting extent_max_zeroout_kb to 0. The downside of this is a potential degredation of a random write workload (using for example the fio benchmark program) on that file system.

As far as I understand what cachefile does, it just truncates a sparse file with a big hole, and do direct IO _only_ all the time to fill the holes.

But the description above is all around (un)written extents, which already have physical blocks allocated, but just without data initialization. So we could zero out the middle extent and merge these extents into one bigger written extent.

However, IMO, it's not the case of what the current cachefiles behavior is... I think rare local fs allocates blocks with direct i/o due to real holes, zero out and merge extents since at least it touches disk quota.

David pointed this message yesterday since we're doing on-demand read feature by using cachefiles as well. But I still fail to understand why the current cachefile behavior is wrong.

Could you kindly leave more hints about this? Many thanks!

Thanks,
Gao Xiang

Quoted 3 lines

Cheers,

- Ted