Chuck Lever wrote:
On Wed, Jul 9, 2008 at 6:30 PM, Shehjar Tikoo <shehjart@xxxxxxxxxxxxxxx> wrote:
Neil Brown wrote:
On Wednesday July 9, shehjart@xxxxxxxxxxxxxxx wrote:
Neil Brown wrote:
So what exactly is this new export option that you want to add?
As the option's name suggests, the idea is to use fallocate support in
ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd sync
writes where each write request has to go to disk almost ASAP. Because NFSv3
writes have to be stable(..not sure about NFSv4..), the write-to-disk and
block allocation must happen immediately. It is possible that the blocks
being allocated for each NFS sync write are not as contiguous as they could
be for say, local buffered writes.
I am hoping that by using some form of adaptive pre-allocation we can
improve the contiguity of disk blocks for nfsd writes.
NFSv3 writes do not have to be stable. The client will usually
request DATA_UNSTABLE, and then send a COMMIT a while later. This
should give the filesystem time to do delayed allocation.
NFSv4 is much the same.
NFSv2 does require stable writes, but it should not be used by anyone
interested in good write performance on large files.
It isn't clear to me that this is something that should be an option
in /etc/exports.
For now, I only need this option so I dont have to rebuild the kernel each
time I want to toggle the "prealloc" option.
When would a sysadmin want to turn it off? Or if a sysadmin did want
control, sure the level of control required would be the size of the
preallocation.
It might be a good idea to turn it off if the block allocation algorithm
slows things down when allocating large number of blocks.
True. If needed, we should be able to add entries in /proc that control min,
max and other limits on preallocation size.
Usually options specific to a particular physical file system are
handled with mount options on the server. NFS export options are used
to tune NFS-specific behavior.
Couldn't you specify a mount option that enables preallocation when
mounting the file system you want to export?
Two points here:
For filesystems that support preallocation,
a) it is already enabled through the inode->i_ops->fallocate operation
while
b) leaving the decision about the size of the pre-allocation up to the
caller, in this case NFS, because the caller will know best about the
pattern of writes it is handing to the filesystem.
So yes, it'll need a NFS level parameter(s), be it an export option or
a module_param.
I can see having a file system callback for the NFS server that
provides a hint that "the client just extended this file and wrote a
bunch of data -- so preallocate blocks for the data, and I will commit
the data at some later point". Most file systems would make this a
no-op.
Ideally, something like preallocation window should become part of the
VFS, like read-ahead data in struct file_ra_state, by adding a
relevant data structure to struct file but that is too big a change at
this point, considering the points below.
But I don't think this would help small synchronous writes... it would
improve block allocation for large writes.
In my really simple prototype, with 64k NFS wsize and a single client
writing a 2Gig file over software raid0, there is no improvement in
write performance for XFS, and a lower throughput figure for ext4 for
all pre-allocation sizes ranging from 5Megs to 100Megs. The read
throughput does improve slightly for ext4. Havent tested reads for XFS
yet. This is for 2.6.25.10. One reason for no performance gain in XFS
could be the fact that the disk in these tests was newly formatted and
the test file was the first file created on the new filesystem so the
blocks allocated in the "no_prealloc" case were mostly contiguous to
begin with(..although that is highly simplifying it..). Perhaps
running a test with multiple writer clients will give more
information. Regarding ext4, I have no idea yet as to why throughput
reduces on using pre-allocation, that too on a fresh filesystem.
I'll run a few more tests in the next few days, in the mean time,
would someone here like to take a look at the patch I have for this
and provide feedback?
Thanks
Shehjar
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