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Add module parameter to disable prefetch in zfs_readdir #16318

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Add module parameter to disable prefetch in zfs_readdir #16318

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18 changes: 18 additions & 0 deletions man/man4/zfs.4
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Original file line number Diff line number Diff line change
Expand Up @@ -1774,6 +1774,24 @@ intact.
Unlike predictive prefetch, prescient prefetch never issues I/O
that ends up not being needed, so it can't hurt performance.
.
.It Sy zfs_readdir_dnode_prefetch_limit Ns = Ns Sy 0 Pq u64
Disable prefetches in readdir for large directories.
When readdir searches a directory, it normally prefetches metadata for
all objects in the directory it checks, even if it's just
looking for a single object.
Setting this to a non-zero value disables that prefetching for directories
with a greater size than that value.
Disabling it for large directories can greatly lower CPU usage on NFS servers
where directories have a very large number of subdirectories.
Directory size in this case is the size returned from calling
.Sy stat
on the directory (stat.st_size).
On ZFS, this directory size value is approximately the number of files
and subdirectories in the directory.
A reasonable value would be 20000.
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To be pedantic, please consider specifying what the units are.

A zero value (the default) means no limit on directory metadata prefetching.
This parameter only applies on Linux.
.
.It Sy zfs_qat_checksum_disable Ns = Ns Sy 0 Ns | Ns 1 Pq int
Disable QAT hardware acceleration for SHA256 checksums.
May be unset after the ZFS modules have been loaded to initialize the QAT
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9 changes: 9 additions & 0 deletions module/os/linux/zfs/zfs_vnops_os.c
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Expand Up @@ -1504,6 +1504,7 @@ zfs_rmdir(znode_t *dzp, char *name, znode_t *cwd, cred_t *cr,
* We use 0 for '.', and 1 for '..'. If this is the root of the filesystem,
* we use the offset 2 for the '.zfs' directory.
*/
static ulong_t zfs_readdir_dnode_prefetch_limit = 0UL;
int
zfs_readdir(struct inode *ip, zpl_dir_context_t *ctx, cred_t *cr)
{
Expand Down Expand Up @@ -1537,6 +1538,9 @@ zfs_readdir(struct inode *ip, zpl_dir_context_t *ctx, cred_t *cr)
os = zfsvfs->z_os;
offset = ctx->pos;
prefetch = zp->z_zn_prefetch;
if (zfs_readdir_dnode_prefetch_limit &&
zp->z_size > zfs_readdir_dnode_prefetch_limit)
prefetch = B_FALSE;

/*
* Initialize the iterator cursor.
Expand Down Expand Up @@ -4252,4 +4256,9 @@ EXPORT_SYMBOL(zfs_map);
/* CSTYLED */
module_param(zfs_delete_blocks, ulong, 0644);
MODULE_PARM_DESC(zfs_delete_blocks, "Delete files larger than N blocks async");

/* CSTYLED */
module_param(zfs_readdir_dnode_prefetch_limit, ulong, 0644);
MODULE_PARM_DESC(zfs_readdir_dnode_prefetch_limit,
"No zfs_readdir prefetch if non-zero and size > this");
#endif
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