Re: [PATCH] NFS: report more appropriate block size for directories.

From: Scott Mayhew
Date: Fri May 08 2015 - 11:15:16 EST

On Fri, 08 May 2015, NeilBrown wrote:

> In glibc 2.21 (and several previous), a call to opendir() will
> result in a 32K (BUFSIZ*4) buffer being allocated and passed to
> getdents.
> However a call to fdopendir() results in an 'fstat' request to
> determine block size and a matching buffer allocated for subsequent
> use with getdents. This will typically be 1M.
> The first getdents call on an NFS directory will always use
> READDIR_PLUS (or NFSv4 equivalent) if available. Subsequent getdents
> calls only use this more expensive version if some 'stat' requests are
> made between the getdents calls.
> For this reason it is good to keep at least that first getdents call
> relatively short. When fdopendir() and readdir() is used on a large
> directory, it takes approximately 32 times as long to complete as
> using "opendir". Current versions of 'find' use fdopendir() and
> demonstrate this slowness.
> 'stat' on a directory currently returns the 'wsize'. This number has
> no meaning on directories.
> Actual READDIR requests are limited to ->dtsize, which itself is
> capped at 4 pages, coincidently the same as BUFSIZ*4.
> So this is a meaningful number to use as the blocksize on directories,
> and has the effect of making 'find' on large directories go a lot
> faster.

Would it make sense to do something similar for regular files too?
fopen() does a similar buffer allocation unless the application
overrides the buffer size via setbuffer()/setvbuf(). That can then
result in fseek() reading a lot of unnecessary data over the wire.

Prior to commit ba52de1 (inode-diet: Eliminate i_blksize from the inode
structure), a stat() over nfs would return the page size in st_blksize,
and for some workloads it does make a difference. For instance, I have
a customer running gdb in an diskless environment. On a stock kernel
where a stat() over nfs returns the wsize in st_blksize, their job takes
~19 minutes... on a test kernel where a stat() over nfs returns the page
size instead, that same job takes ~13 minutes. I hadn't sent a patch
yet because I'm still trying to account for a few extra minutes of
run time elsewhere...

> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
> index 96f2d55781fb..f8aebf59383f 100644
> --- a/fs/nfs/inode.c
> +++ b/fs/nfs/inode.c
> @@ -678,6 +678,8 @@ int nfs_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
> if (!err) {
> generic_fillattr(inode, stat);
> stat->ino = nfs_compat_user_ino64(NFS_FILEID(inode));
> + if (S_ISDIR(inode->i_mode))
> + stat->blksize = NFS_SERVER(inode)->dtsize;
> }
> out:
> trace_nfs_getattr_exit(inode, err);
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