From: Eric Sandeen <sandeen@sandeen.net>
To: xfs@oss.sgi.com, Jim Wilcoxson <prirun@gmail.com>
Subject: Re: XFS fiemap issue with Linux 3.10.0-327.el7.x86_64 (CentOS 7)
Date: Mon, 15 Feb 2016 12:33:20 -0600 [thread overview]
Message-ID: <56C219F0.3050002@sandeen.net> (raw)
In-Reply-To: <CAJkH1p6LU=d5oLh3A+j_GQ70uoCYMjR=a5yS1w8p5d5ZXz6fwg@mail.gmail.com>
On 2/15/16 11:26 AM, Jim Wilcoxson wrote:
> I'm developing a backup program, HashBackup, and recently added sparse
> "hole skipping" using fiemap.
That is probably not safe. Mapping can change immediately after fiemap
returns, and you could miss data. cp went down that path a few years ago,
and it caused pain.
SEEK_HOLE/SEEK_DATA would be a better choice.
> Today I ran into a weird issue with xfs
> on Linux 3.10.
>
> A test program creates a sparse file with 10000*(4K data, 4M hole).
> HB calls fiemap with start=0, length=0xFFFF...FFFF, mapped_extents=0
> to get the number of extents. Fiemap is coming back with
> mapped_extents=1364 instead of 10000.
Take a look at xfs_bmap -v output to get a clear picture of what
is actually on disk. It's possible that some combination of
speculative preallocation or other things has merged extents.
In general, the kernel has plenty of leeway in terms of how it
chooses to lay out a file. "seek/write" doesn't necessarily
guarantee a hole.
> The fiemap.txt file says:
>
> "If fm_extent_count is zero, then the fm_extents[] array is ignored
> (no extents will be returned), and the fm_mapped_extents count will
> hold the number of extents needed in fm_extents[] to hold the file's
> current mapping."
>
> It doesn't say the filesystem can choose to return fewer extents if it
> wants, but maybe xfs interprets it this way and fiemap has to be
> called in a loop until the extent_last flag is set. If that's the
> case, fiemap.txt should be updated.
It's not clear that it is returning fewer extents. See what xfs_bmap
says, and compare.
But also, don't use fiemap results for anything that requires
data integrity...
-Eric
> Thanks,
> Jim
> (not on the xfs mailing list)
>
_______________________________________________
xfs mailing list
xfs@oss.sgi.com
http://oss.sgi.com/mailman/listinfo/xfs
next prev parent reply other threads:[~2016-02-15 18:33 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-15 17:26 XFS fiemap issue with Linux 3.10.0-327.el7.x86_64 (CentOS 7) Jim Wilcoxson
2016-02-15 18:33 ` Eric Sandeen [this message]
2016-02-15 19:28 ` Jim Wilcoxson
2016-02-15 20:40 ` Dave Chinner
2016-02-15 22:47 ` Eric Sandeen
2016-02-16 0:20 ` Dave Chinner
2016-02-16 0:23 ` Eric Sandeen
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=56C219F0.3050002@sandeen.net \
--to=sandeen@sandeen.net \
--cc=prirun@gmail.com \
--cc=xfs@oss.sgi.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.