From 416e232cc6d1b5a198023be3602296f582768839 Mon Sep 17 00:00:00 2001 From: Bruce Evans Date: Fri, 21 Dec 2018 21:17:45 +0000 Subject: [PATCH] Fix clobbering of the fatchain cache for clustered i/o's when full 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. --- sys/fs/msdosfs/msdosfs_vnops.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/sys/fs/msdosfs/msdosfs_vnops.c b/sys/fs/msdosfs/msdosfs_vnops.c index adf1e402c15..0aaf4bddb80 100644 --- a/sys/fs/msdosfs/msdosfs_vnops.c +++ b/sys/fs/msdosfs/msdosfs_vnops.c @@ -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); } -- 2.45.0