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Prefetch start and end of volumes

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When adding a zvol to the system prefetch zvol_prefetch_bytes from the
start and end of the volume.  Prefetching these regions of the volume is
desirable because they are likely to be accessed immediately by blkid(8),
the kernel scanning for a partition table, or another task which probes
the devices.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3659
This commit is contained in:
Brian Behlendorf 2015-08-18 13:51:20 -07:00
parent 8198d18ca7
commit 9965059ab9
2 changed files with 32 additions and 0 deletions
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15
man/man5/zfs-module-parameters.5
View file

@ -1631,6 +1631,21 @@ Max number of blocks to discard at once
Default value: \fB16,384\fR.
.RE
.sp
.ne 2
.na
\fBzvol_prefetch_bytes\fR (uint)
.ad
.RS 12n
When adding a zvol to the system prefetch \fBzvol_prefetch_bytes\fR
from the start and end of the volume. Prefetching these regions
of the volume is desirable because they are likely to be accessed
immediately by \fBblkid(8)\fR or by the kernel scanning for a partition
table.
.sp
Default value: \fB131,072\fR.
.RE
.SH ZFS I/O SCHEDULER
ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os.
The I/O scheduler determines when and in what order those operations are

17
module/zfs/zvol.c
View file

@ -50,6 +50,7 @@
unsigned int zvol_inhibit_dev = 0;
unsigned int zvol_major = ZVOL_MAJOR;
unsigned int zvol_prefetch_bytes = (128 * 1024);
unsigned long zvol_max_discard_blocks = 16384;
static kmutex_t zvol_state_lock;
@ -1296,6 +1297,7 @@ __zvol_create_minor(const char *name, boolean_t ignore_snapdev)
objset_t *os;
dmu_object_info_t *doi;
uint64_t volsize;
uint64_t len;
unsigned minor = 0;
int error = 0;
@ -1369,6 +1371,18 @@ __zvol_create_minor(const char *name, boolean_t ignore_snapdev)
zil_replay(os, zv, zvol_replay_vector);
}
/*
* When udev detects the addition of the device it will immediately
* invoke blkid(8) to determine the type of content on the device.
* Prefetching the blocks commonly scanned by blkid(8) will speed
* up this process.
*/
len = MIN(MAX(zvol_prefetch_bytes, 0), SPA_MAXBLOCKSIZE);
if (len > 0) {
dmu_prefetch(os, ZVOL_OBJ, 0, len);
dmu_prefetch(os, ZVOL_OBJ, volsize - len, len);
}
zv->zv_objset = NULL;
out_dmu_objset_disown:
dmu_objset_disown(os, zvol_tag);
@ -1625,3 +1639,6 @@ MODULE_PARM_DESC(zvol_major, "Major number for zvol device");
module_param(zvol_max_discard_blocks, ulong, 0444);
MODULE_PARM_DESC(zvol_max_discard_blocks, "Max number of blocks to discard");
module_param(zvol_prefetch_bytes, uint, 0644);
MODULE_PARM_DESC(zvol_prefetch_bytes, "Prefetch N bytes at zvol start+end");