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nfs performance delta between filesystems

DISCUSSION Emmanuel Florac 4 participants 7 messages Jan 22, 2010 – Jan 25, 2010

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Emmanuel Florac

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#0 lore raw

Here is an interesting mystery to solve : XFS is always much faster locally than all other FSs, but quite slower through NFS.

I'm benchmarking a RAID-10 setup with various filesystems, ext3, reiser (3.6), jfs and xfs. I'm strictly and only interested in continuous sequential write (data recording) through NFS (basic 1G Eth), so I'm benchmarking with

dd if=/dev/zero of=/mnt/whatever/file bs=1M

following the throughput with iostat.

Setup : linux 2.6.27.21, software RAID 10 (64K stripe), 1GB RAM, 64 bits kernel, 4 Savvio 500GB disks, Phenom II, Intel Pro1000 network.

All the following tests have been run several times each, to even results.

Locally, xfs mounted this way

/dev/md2 on /mnt/raid type xfs (rw,noatime,nobarrier)

is wayyyyy better than all other filesystems :

~# dd if=/dev/zero of=/mnt/raid/testdd bs=1M count=4096 4096+0 records in
4096+0 records out
4294967296 bytes (4.3 GB) copied, 23.0655 s, 186 MB/s

ext3, jfs and reiser score the same around 150 MB/s.

I'm exporting the mountpoint through NFS :

/mnt/raid *(fsid=0,rw,async,no_root_squash,no_subtree_check)

And mount with no options (or just "tcp") on the client. Then the results are quite different. XFS gives a bit less than 90 MB/s (not bad):

~$ sudo dd if=/dev/zero of=/mnt/temp/testnfsdd1 bs=1M count=2048
2048+0 enregistrements lus
2048+0 enregistrements écrits
2147483648 octets (2,1 GB) copiés, 24,1755 s, 88,8 MB/s

But... reiserfs gives ~ 92 MB/s, and ext3 95 MB/s. Results are consistently better with both through NFS, while they're constantly much worse locally... How come???

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Emmanuel Florac     |   Direction technique
                    |   Intellique
                    |	<[email protected]>
                    |   +33 1 78 94 84 02
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#1 lore raw

Hey Emmanuel,

I did some research on this in April last year on an old, old kernel. One of the codepaths I flagged:

nfsd_create
write_inode_now
__sync_single_inode
write_inode
xfs_fs_write_inode
xfs_inode_flush
xfs_iflush

There were small gains to be had by reordering the sync of the parent and child syncs where the two inodes were in the same cluster. The larger problem seemed to be that we're not treating the log as stable storage. By calling write_inode_now we've written the changes to the log first and then gone and also written them out to the inode.

nfsd_create, nfsd_link, and nfsd_setattr all do this (or do in the old kernel I'm looking at). I have a patchset that changes
this to an fsync so we force the log and call it good. I'll be happy to dust it off if someone hasn't already addressed this situation.

-Ben

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#2 lore raw

Le Fri, 22 Jan 2010 12:38:48 -0600 vous écriviez:

Quoted 5 lines

There were small gains to be had by reordering the sync of the parent and child syncs where the two inodes were in the same cluster. The larger problem seemed to be that we're not treating the log as stable storage. By calling write_inode_now we've written the changes to the log first and then gone and also written them out to the inode.

I thought that using nobarrier would prevent the log operations to be sync'd to disk too quickly? Could I possibly enhance this behaviour by using an external log on a very fast storage (ramdisk...)?

Quoted 4 lines

nfsd_create, nfsd_link, and nfsd_setattr all do this (or do in the old kernel I'm looking at). I have a patchset that changes this to an fsync so we force the log and call it good. I'll be happy to dust it off if someone hasn't already addressed this situation.

Well, if you think this patchset can be applied to a current kernel without too much hacking I'm ready to give it a try :)

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Emmanuel Florac     |   Direction technique
                    |   Intellique
                    |	<[email protected]>
                    |   +33 1 78 94 84 02
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#3 lore raw
In reply to [email protected]
Quoted 13 lines · On Fri, Jan 22, 2010 at 12:38:48PM -0600, [email protected] wrote:

Hey Emmanuel,

I did some research on this in April last year on an old, old kernel. One of the codepaths I flagged:

nfsd_create
write_inode_now
__sync_single_inode
write_inode
xfs_fs_write_inode
xfs_inode_flush
xfs_iflush

Quoted 5 lines

There were small gains to be had by reordering the sync of the parent and child syncs where the two inodes were in the same cluster. The larger problem seemed to be that we're not treating the log as stable storage. By calling write_inode_now we've written the changes to the log first and then gone and also written them out to the inode.

Pretty much right, but there are historical reasons for that behaviour. The ->write_inode() path is the only
method for the higher layers to say "write this inode to disk". That's how XFS has been treating it for a long time - as a command to _physically_ write a dirty inode some time after it was first changed and the transaction is already on disk.

Unfortunately, NFS is using the same call for is a method for saying "commit this changed inode to disk immediately", which is a different semantic to the way the sync code uses it and physical inode IO really hurts here.

Quoted 4 lines

nfsd_create, nfsd_link, and nfsd_setattr all do this (or do in the old kernel I'm looking at). I have a patchset that changes
this to an fsync so we force the log and call it good. I'll be happy to dust it off if someone hasn't already addressed this situation.

The delayed write inode flushing patchset I'm finalising does this. We now have reliable tracking of dirty inodes in XFS and a method for efficient physical writeback, so we no longer need to rely on ->write_inode to tell us to write inodes to disk. Hence the patchset turns the inode write into a an xfs_fsync() if it is a sync write or a delayed write if it is async. I'm hoping to have that ready for .34 inclusion sometime next week...

Cheers,

Dave.

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Dave Chinner
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#4 lore raw
In reply to [email protected]
Quoted 19 lines · On Fri, Jan 22, 2010 at 12:38:48PM -0600, [email protected] wrote:

Hey Emmanuel,

I did some research on this in April last year on an old, old kernel. One of the codepaths I flagged:

nfsd_create
write_inode_now
__sync_single_inode
write_inode
xfs_fs_write_inode
xfs_inode_flush
xfs_iflush

There were small gains to be had by reordering the sync of the parent and child syncs where the two inodes were in the same cluster. The larger problem seemed to be that we're not treating the log as stable storage. By calling write_inode_now we've written the changes to the log first and then gone and also written them out to the inode.

Quoted 4 lines

nfsd_create, nfsd_link, and nfsd_setattr all do this (or do in the old kernel I'm looking at). I have a patchset that changes
this to an fsync so we force the log and call it good. I'll be happy to dust it off if someone hasn't already addressed this situation.

Dave and I had had some discussion about this when going through his inode writeback changes. Changing to ->fsync might indeed be the easiest option, but on the other hand I'm really trying to get rid of the special case of ->fsync without a file argument in the VFS as it complicates stackable filesystem layers and also creates a rather annoying and under/un documented assumtion that filesystem that need the file pointer can't be NFS exported. One option if we want to keep these semantics is to add a new export operation just for synchronizations things in NFS.

But given that the current use case in NFS is to pair one write_inode call with one actual VFS operation it might be better to just automatically turn on the wsync mount option in XFS, we'd need a hook from NFSD into the filesystem to implement this, but I've been looking into adding this anyway to allow for checking other paramters like the file handle size against filesystem limitations. Any chance you could run your tests against a wsync filesystem?

But all this affects metadata performance, and only for sync exports, while the OP does a simple dd which is streaming data I/O and uses the (extremly unsafe) async export operation that disables the write_inode calls.

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#5 lore raw
Quoted 35 lines · Christoph Hellwig
Quoted 19 lines · On Fri, Jan 22, 2010 at 12:38:48PM -0600, [email protected] wrote:

Hey Emmanuel,

I did some research on this in April last year on an old, old kernel. One of the codepaths I flagged:

nfsd_create
write_inode_now
__sync_single_inode
write_inode
xfs_fs_write_inode
xfs_inode_flush
xfs_iflush

There were small gains to be had by reordering the sync of the parent and child syncs where the two inodes were in the same cluster. The larger problem seemed to be that we're not treating the log as stable storage. By calling write_inode_now we've written the changes to the log first and then gone and also written them out to the inode.

Quoted 4 lines

nfsd_create, nfsd_link, and nfsd_setattr all do this (or do in the old kernel I'm looking at). I have a patchset that changes
this to an fsync so we force the log and call it good. I'll be happy to dust it off if someone hasn't already addressed this situation.

Dave and I had had some discussion about this when going through his inode writeback changes. Changing to ->fsync might indeed be the easiest option, but on the other hand I'm really trying to get rid of the special case of ->fsync without a file argument in the VFS as it complicates stackable filesystem layers and also creates a rather annoying and under/un documented assumtion that filesystem that need the file pointer can't be NFS exported. One option if we want to keep these semantics is to add a new export operation just for synchronizations things in NFS.

Quoted 7 lines

But given that the current use case in NFS is to pair one write_inode call with one actual VFS operation it might be better to just automatically turn on the wsync mount option in XFS, we'd need a hook from NFSD into the filesystem to implement this, but I've been looking into adding this anyway to allow for checking other paramters like the file handle size against filesystem limitations. Any chance you could run your tests against a wsync filesystem?

The original tests were done with the wsync mount option. I'm not really sure that it was necessary. Test case was "tar -xvf ImageMagick.tar". 'fdatasync' represents whether the export option controlling usage of write_inode_now vs fsync was set.

internal log, no wsync, no fdatasync
2m48.632s 2m59.676s 2m42.450s

internal log, wsync, no fdatasync
3m1.320s 3m10.961s 2m53.560s

internal log, wsync, fdatasync
1m40.191s 1m38.780s 1m35.758s

external log, no wsync, no fdatasync
1m37.069s 1m37.850s 1m38.303s

external log, wsync, no fdatasync
1m48.948s 1m47.804s 1m50.219s

external log, wsync, fdatasync
1m19.265s 1m19.129s 1m19.635s

Quoted 4 lines

But all this affects metadata performance, and only for sync exports, while the OP does a simple dd which is streaming data I/O and uses the (extremly unsafe) async export operation that disables the write_inode calls.

Right. This might not apply to Emmanuel's problem. I've been wondering if a recent change to not hold the inode mutex over the sync helps in the streaming io case. Any idea?

Anyway, looks like Dave's patchset addresses this.

-Ben

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#6 lore raw
Quoted 4 lines · On Mon, Jan 25, 2010 at 02:28:39PM -0600, [email protected] wrote:

The original tests were done with the wsync mount option. I'm not really sure that it was necessary. Test case was "tar -xvf ImageMagick.tar". 'fdatasync' represents whether the export option controlling usage of write_inode_now vs fsync was set.

Ok. Btw, you need to call ->fsync with fdatasync = 0 for NFS as it also wants to catch non-data changes to the inode. Doesn't matter for XFS as we currently always force a full fsync, but I'm going to change that soon.

Quoted 8 lines

internal log, no wsync, no fdatasync
2m48.632s 2m59.676s 2m42.450s

internal log, wsync, no fdatasync
3m1.320s 3m10.961s 2m53.560s

internal log, wsync, fdatasync
1m40.191s 1m38.780s 1m35.758s

The wsync case always still includes either the ->fsync or write_inode call, right? If we use wsync we shouldn't need either in theory as the transactions already commit synchronously.

Anyway, given the massive improvements of ->fsync vs write_inode you really should post that patch to the NFS list for discussion ASAP.

Quoted 8 lines
Quoted 4 lines

But all this affects metadata performance, and only for sync exports, while the OP does a simple dd which is streaming data I/O and uses the (extremly unsafe) async export operation that disables the write_inode calls.

Right. This might not apply to Emmanuel's problem. I've been wondering if a recent change to not hold the inode mutex over the sync helps in the streaming io case. Any idea?

It should help a bit. I'm not sure it can cause that much of a difference for such a simple single-threaded workload. Emmanuel, is there any chance you could try the latest 2.6.32-stable kernel or even 2.6.33-rc as those changes are included there?

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