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Re: XFS Preallocate using ALLOCSP

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: XFS Preallocate using ALLOCSP
From: Felix Blyakher <felixb@xxxxxxx>
Date: Tue, 16 Jun 2009 11:42:01 -0500
Cc: Smit Shah <getsmit@xxxxxxxxx>, linux-xfs@xxxxxxxxxxx
In-reply-to: <4A37B744.9030301@xxxxxxxxxxx>
References: <24042506.post@xxxxxxxxxxxxxxx> <4A3712BF.7030101@xxxxxxxxxxx> <8770d98c0906152344p185533a9rc144a5667d13d2de@xxxxxxxxxxxxxx> <4A37B744.9030301@xxxxxxxxxxx>

On Jun 16, 2009, at 10:16 AM, Eric Sandeen wrote:

Smit Shah wrote:

Even the man page of fallocate says that it allocates and initializes
to zero the disk space allocated

Bleah, so it does:

This flag allocates and initializes to zero the disk space

well, that's misleading and/or wrong.

but when i saw the code i did found out that it does not zero it out.
Hence was a kindof confused. So posix_allocate is similar to ALLOCSP
when falloc is not supported by the underlying filesystem  that is to
ftruncate the file  and zero it out. So all of them try to allocate
contiguous blocks but the only difference is when we use the
fallocate in ext4/xfs it does not zero out the preallocated space. Am
i right ?

fallocate / sys_fallocate marks the region as uninitialized so that you get back 0s when you read. It's implemented on xfs, ext4, ocfs2, and btrfs.

posix_fallocate manages to reach sys_fallocate when all the stars align:
kernel,  glibc, and filesystgem.  Otherwise it writes 0s.

But  when i fallocate in ext4 i can see the write performance
improvement but not in xfs

Testing how?

and reason i found out in one of your previous comments is  because
of the unwritten flag set in xfs. So how do we see if the unwritten
flag is set or not ? I did use xfs_info but it didnt show any such

ext4 & xfs are doing the same basic thing, they must maintain the
unwritten state on the preallocated extents, and manage that as it
changes when portions are written with real data.

Well, the difference in managing the unwritten state can
theoretically result in different performance. Not that I'd
expect ext4 being better than xfs in this respect.
More data is needed here.

xfs_bmap -v -v -p on a file will show you extent state for xfs.

I guess i am not right here ftruncate simply does a lseek and wirtes

ftruncate simply sets i_size, it does no data IO.

... and no block reservation/allocation either.

to it which might not be contiguous whereas fallocate tries to
allocate contiguous block so as to reduce fragmentation

Actually fallocate's only official job is to reserve blocks so you don't
get ENOSPC later.  Because the request comes in all at once, you are
very likely to get an optimal allocation, and that's a nice side effect,
but it's not actually required by the interface.

and hence i
thought to reduce fragmentation and for security reasons

None of these normal interfaces poses any security risk.  If you build
xfs without the unwritten extent feature

I don't think, it's possible. Not in any configurable way,
at least.

you could allocate w/o flagging
uninitialized and expose stale data, but that's not a normal mode of

That's was possible with mount option unwritten=0, but
AFAIK, it's been recently completely removed from code.

its better
to use ALLOCSP rather than something like ftruncate /posix_fallocate
or RSEVSP which kindof performs bad for writes with unwritten flag
set and now there being a no direct way while creating the fs to
disable unwritten.

In the end, there are only 2 ways to preallocate blocks: explicitly
write 0s, or flag regions as unwritten (as xfs/ext4/... can do).

That's a trade-off between spending time on setup or at the
write time. And if explicit zeros are desirable for the former
approach, it can be driven from the user space (after
preallocation) rather than from the kernel with the exactly the
same outcome.

Just restating the same what Eric already said :)


or a 3rd sorta-way, which is to reserve w/o flagging, maybe that's what
you're looking for, but that's deprecated or not really available at
this point).

Maybe I should ask what the end goal is here.  :)


Thanks, Smit

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