Hello,

 

just a short followup. It seems I found the issue causing our file system cache to fill up during backup jobs and our virtual platform memory usage to increase.

 

The O_RDONLY mask is “00” on Linux, casing the if flags & O_RDONLY to always fail. So the patch regarding posix_fadvise() in commit 4df486dd5b04cc16178efcb75eaf3dbd4be44b31 is actually dead code. (if (Anything & 00) { dead code; })

 

This diff fixes the issue:

 

diff --git a/bacula/src/findlib/bfile.c b/bacula/src/findlib/bfile.c

index e315675..29f04a3 100644

--- a/bacula/src/findlib/bfile.c

+++ b/bacula/src/findlib/bfile.c

@@ -997,9 +997,9 @@ int bopen(BFILE *bfd, const char *fname, int flags, mode_t mode)

    bfd->win32DecompContext.liNextHeader = 0;

 

#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_WILLNEED)

-   if (bfd->fid != -1 && flags & O_RDONLY) {

+   if (bfd->fid != -1 && flags == O_RDONLY) {

       int stat = posix_fadvise(bfd->fid, 0, 0, POSIX_FADV_WILLNEED);

-      Dmsg2(400, "Did posix_fadvise on %s stat=%d\n", fname, stat);

+      Dmsg2(400, "Did posix_fadvise POSIX_FADV_WILLNEED on %s stat=%d\n", fname, stat);

    }

#endif

 

@@ -1043,10 +1043,11 @@ int bclose(BFILE *bfd)

       return 0;

    }

#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)

-   if (bfd->m_flags & O_RDONLY) {

+   if (bfd->m_flags == O_RDONLY) {

       fdatasync(bfd->fid);            /* sync the file */

       /* Tell OS we don't need it any more */

       posix_fadvise(bfd->fid, 0, 0, POSIX_FADV_DONTNEED);

+      Dmsg1(400, "Did posix_fadvise POSIX_FADV_DONTNEED on File Desriptor %d\n", bfd->fid);

    }

#endif

 

After applying this patch, the cache memory does not increase a lot during backups on my test machine. Running widh bacula-fd –d 400 also shows the debugging messages.

 

However this fix has a HUGE impact on behavior of existing backup jobs - altought it has huge benefits on virtual platforms with short backup windows also.

 

Current behavior:

-          backup jobs with snapshots: caching of unneeded files in memory, filesystem cache is polluted, virtual platform memory usage increases

-          backup Jobs without snapshots: backup job warms up the cache of all files

 

Behaviour with fix:

-          backup jobs with snapshots: only caching of 1 unneeded file in memory, filesystem cache is less polluted (largest file to backup), virtual platform memory usage decreases

-          backup Jobs without snapshots: backup job does not warms up the cache of all files, but also drops existing cache for cached files (may have performance impact for webservers, fileservers etc.)

 

 

Conclusion:

-          As the impact on behavior is huge, fixing the bug in place can be problematic

-          Unfortunately the right way to impelement posix_fadvise() with POSIX_FADV_NOREUSE is not implemented on Linux (See http://lxr.free-electrons.com/source/mm/fadvise.c#L115), so there is “right” way to fix this issue

-          Best approach: there should be a Bacula Option to enable the POSIX_FADV_NOREUSE explicitly for selective bacula Jobs to allow usage of posix_fadvise() for direct I/O workloads (like mysql  databases etc with snapshots etc.) and avoid it for webserver and fileserver workloads. Admins can decide. The default should be the current behavior to not call posix_fadvise(). Also the code path for SD/Dir und FD should be separate so enable this for the FD bfile.c code only (it seems currently it is the same code).

 

Regards,

Robert

 

 

 

Hello,

 

we are using bacula on a virtual plattform (> 600 clients) and investiate moderate swap usage issues / cache usage issues. It seems that during backup of mysql datafiles (LVM Snapshot), the “cached” memory usage of the server increases and thus bacula backup reads (only done one) are cached (… polluting the rest of the vfs cache ?)

 

From reading the source of src/findlib/bfile.c, bacula supports posix_fadvise for a long time now (2007), by those two code pices:

 

commit 4df486dd5b04cc16178efcb75eaf3dbd4be44b31

Author: Kern Sibbald <kern AT sibbald DOT com>

Date:   Thu May 24 19:58:07 2007 +0000

 

    kes  Add code to tell the OS that we no longer need a cached

         file that we were reading. In findlib/bfile.c.  Also,

         only cache files that we are reading.

    kes  Tweak to bsmtp to eliminate compiler warnings on Win32.

    kes  Implement script to automatically generate cats and dll .def

         files for Win32 dll.

    kes  Update README.mingw32 to include new .def file generation.

 

    git-svn-id: https://bacula.svn.sourceforge.net/svnroot/bacula/trunk@4898 91ce42f0-d328-0410-95d8-f526ca767f89

 

bopen()

 

#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_WILLNEED)

   if (bfd->fid != -1 && flags & O_RDONLY) {

      int stat = posix_fadvise(bfd->fid, 0, 0, POSIX_FADV_WILLNEED);

      Dmsg2(400, "Did posix_fadvise on %s stat=%d\n", fname, stat);

   }

#endif

 

bclose()

 

#if defined(HAVE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)

   if (bfd->m_flags & O_RDONLY) {

      fdatasync(bfd->fid);            /* sync the file */

      /* Tell OS we don't need it any more */

      posix_fadvise(bfd->fid, 0, 0, POSIX_FADV_DONTNEED);

   }

#endif

 

I checked that our bacula client was compiled with HAVE_POSIX_FADVISE (added debug messages).

 

I also added more debugging code and it seems that during a normal backup run, the code is never executed on bacula-fd as O_RDONLY is not set in the flags of the originating bopen request.

 

Is this a bug or a feature ?

 

How can I prevent cache usage of bacula-fd on the client side at all ? Is there a Flag in bacula-fd to force O_READONLY opening of backed up files ?

 

Regards,

Robert