Illumos 5027 - zfs large block support

5027 zfs large block support
Reviewed by: Alek Pinchuk <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Josef 'Jeff' Sipek <[email protected]>
Reviewed by: Richard Elling <[email protected]>
Reviewed by: Saso Kiselkov <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Approved by: Dan McDonald <[email protected]>

References:
  https://www.illumos.org/issues/5027
  https://github.com/illumos/illumos-gate/commit/b515258

Porting Notes:

* Included in this patch is a tiny ISP2() cleanup in zio_init() from
Illumos 5255.

* Unlike the upstream Illumos commit this patch does not impose an
arbitrary 128K block size limit on volumes.  Volumes, like filesystems,
are limited by the zfs_max_recordsize=1M module option.

* By default the maximum record size is limited to 1M by the module
option zfs_max_recordsize.  This value may be safely increased up to
16M which is the largest block size supported by the on-disk format.
At the moment, 1M blocks clearly offer a significant performance
improvement but the benefits of going beyond this for the majority
of workloads are less clear.

* The illumos version of this patch increased DMU_MAX_ACCESS to 32M.
This was determined not to be large enough when using 16M blocks
because the zfs_make_xattrdir() function will fail (EFBIG) when
assigning a TX.  This was immediately observed under Linux because
all newly created files must have a security xattr created and
that was failing.  Therefore, we've set DMU_MAX_ACCESS to 64M.

* On 32-bit platforms a hard limit of 1M is set for blocks due
to the limited virtual address space.  We should be able to relax
this one the ABD patches are merged.

Ported-by: Brian Behlendorf <[email protected]>
Closes #354

1 files changed, 18 insertions, 0 deletions

diff --git a/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5
index 4bbfbf512..359e9f72f 100644
--- a/man/man5/zfs-module-parameters.5
+++ b/man/man5/zfs-module-parameters.5
@@ -948,6 +948,24 @@ Default value: \fB32,768\fR.
 .sp
 .ne 2
 .na
+\fBzfs_max_recordsize\fR (int)
+.ad
+.RS 12n
+We currently support block sizes from 512 bytes to 16MB.  The benefits of
+larger blocks, and thus larger IO, need to be weighed against the cost of
+COWing a giant block to modify one byte.  Additionally, very large blocks
+can have an impact on i/o latency, and also potentially on the memory
+allocator.  Therefore, we do not allow the recordsize to be set larger than
+zfs_max_recordsize (default 1MB).  Larger blocks can be created by changing
+this tunable, and pools with larger blocks can always be imported and used,
+regardless of this setting.
+.sp
+Default value: \fB1,048,576\fR.
+.RE
+
+.sp
+.ne 2
+.na
 \fBzfs_mdcomp_disable\fR (int)
 .ad
 .RS 12n