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Fix clobbering of the fatchain cache for clustered i/o's when full

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clustering is not done.  The bug caused extreme slowness for large
files in some cases.

There is no way to tell VOP_BMAP() how many blocks are wanted, so for
all file systems it has to waste time in some cases by searching for
more contiguous blocks than will be accessed.  For msdosfs, it also
clobbered the fatchain cache in these cases by advancing the cache to
point to the chain entry for block that won't be read.  This makes
the cache useless for the next sequential i/o (or VOP_BMAP()), so the
fat chain is searched from the beginning.  The cache only has 1 relevant
entry, so it is similarly useless for random i/o.

Fix this by only advancing the cache to point to the chain entry for
the first block that will be read.  Clustering uses results from
VOP_BMAP(), so when more than 1 block is read by clustering, the cache
is not advanced as optimally as before, but it is at most 1 cluster
size behind and searching the chain through the blocks for this cluster
doesn't take too long.
This commit is contained in:
Bruce Evans 2018-12-21 21:17:45 +00:00
parent 6c5c0137a9
commit 416e232cc6
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=342357
1 changed files with 16 additions and 0 deletions
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16
sys/fs/msdosfs/msdosfs_vnops.c
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@ -1740,6 +1740,7 @@ msdosfs_readdir(struct vop_readdir_args *ap)
static int
msdosfs_bmap(struct vop_bmap_args *ap)
{
struct fatcache savefc;
struct denode *dep;
struct mount *mp;
struct msdosfsmount *pmp;
@ -1766,6 +1767,20 @@ msdosfs_bmap(struct vop_bmap_args *ap)
if (error != 0 || (ap->a_runp == NULL && ap->a_runb == NULL))
return (error);
/*
* Prepare to back out updates of the fatchain cache after the one
* for the first block done by pcbmap() above. Without the backout,
* then whenever the caller doesn't do i/o to all of the blocks that
* we find, the single useful cache entry would be too far in advance
* of the actual i/o to work for the next sequential i/o. Then the
* FAT would be searched from the beginning. With the backout, the
* FAT is searched starting at most a few blocks early. This wastes
* much less time. Time is also wasted finding more blocks than the
* caller will do i/o to. This is necessary because the runlength
* parameters are output-only.
*/
savefc = dep->de_fc[FC_LASTMAP];
mp = vp->v_mount;
maxio = mp->mnt_iosize_max / mp->mnt_stat.f_iosize;
bnpercn = de_cn2bn(pmp, 1);
@ -1787,6 +1802,7 @@ msdosfs_bmap(struct vop_bmap_args *ap)
}
*ap->a_runb = run - 1;
}
dep->de_fc[FC_LASTMAP] = savefc;
return (0);
}