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Re: [PATCH] Increase the default size of the reserved blocks pool

To: Lachlan McIlroy <lachlan@xxxxxxx>
Subject: Re: [PATCH] Increase the default size of the reserved blocks pool
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 30 Sep 2008 14:11:49 +1000
Cc: xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <48E097B5.3010906@xxxxxxx>
Mail-followup-to: Lachlan McIlroy <lachlan@xxxxxxx>, xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
References: <48E097B5.3010906@xxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
On Mon, Sep 29, 2008 at 06:54:13PM +1000, Lachlan McIlroy wrote:
> The current default size of the reserved blocks pool is easy to deplete
> with certain workloads, in particular workloads that do lots of concurrent
> delayed allocation extent conversions.  If enough transactions are running
> in parallel and the entire pool is consumed then subsequent calls to
> xfs_trans_reserve() will fail with ENOSPC.  Also add a rate limited
> warning so we know if this starts happening again.
>
> --- a/fs/xfs/xfs_mount.c      2008-09-29 18:30:26.000000000 +1000
> +++ b/fs/xfs/xfs_mount.c      2008-09-29 18:27:37.000000000 +1000
> @@ -1194,7 +1194,7 @@ xfs_mountfs(
>        */
>       resblks = mp->m_sb.sb_dblocks;
>       do_div(resblks, 20);
> -     resblks = min_t(__uint64_t, resblks, 1024);
> +     resblks = min_t(__uint64_t, resblks, 16384);

I'm still not convinced such a large increase is needed for average
case. This means that at a filesystem size of 5GB we are reserving
256MB (5%) for a corner case workload that is unlikely to be run on a
5GB filesystem. That is a substantial reduction in space for such
a filesystem, and quite possibly will drive systems into immediate
ENOSPC at mount. At that point stuff is going to fail badly during
boot.

Indeed - this will ENOSPC the root drive on my laptop the moment I
apply it (6GB root, 200MB free) and reboot, as well as my main
server (4GB root - 150MB free, 2GB /var - 100MB free, etc).
On that basis alone, I'd suggest this is a bad change to make to the
default value of the reserved block pool.

>       error = xfs_reserve_blocks(mp, &resblks, NULL);
>       if (error)
>               cmn_err(CE_WARN, "XFS: Unable to allocate reserve blocks. "
> @@ -1483,6 +1483,7 @@ xfs_mod_incore_sb_unlocked(
>       int             scounter;       /* short counter for 32 bit fields */
>       long long       lcounter;       /* long counter for 64 bit fields */
>       long long       res_used, rem;
> +     static int      depleted = 0;
>
>       /*
>        * With the in-core superblock spin lock held, switch
> @@ -1535,6 +1536,9 @@ xfs_mod_incore_sb_unlocked(
>                               if (rsvd) {
>                                       lcounter = (long 
> long)mp->m_resblks_avail + delta;
>                                       if (lcounter < 0) {
> +                                             if ((depleted % 100) == 0)
> +                                                     printk(KERN_DEBUG "XFS 
> reserved blocks pool depleted.\n");
> +                                             depleted++;
>                                               return XFS_ERROR(ENOSPC);
>                                       }

This should use the generic printk ratelimiter, and the error message
should use xfs_fs_cmn_err() to indicate what filesystem the error
is occuring on. ie.:

        if (printk_ratelimit())
                xfs_fs_cmn_err(CE_WARN, mp,
                                "ENOSPC: reserved block pool empty");

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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