account for ashift when choosing buffers to be written to l2arc device

Review Request #229 - Created June 16, 2015 and submitted

Andriy Gapon
gibbs, gwilson, mahrens, skiselkov

NOTE: this review request supercedes #112.

If we don't account for that, then we might end up overwriting disk
area of buffers that have not been evicted yet, because l2arc_evict
operates in terms of disk addresses.

The discrepancy between the write size calculation and the actual increment
to l2ad_hand was introduced in
commit aad02571bc59671aa3103bb070ae365f531b0b62

The change that introduced l2ad_hand alignment was almost correct
as the write size was accumulated as a sum of rounded buffer sizes.
See commit e14bb3258d05c1b1077e2db7cf77088924e56919

Also, we now consistently use asize / a_sz for the allocated size and
psize / p_sz for the physical size.
The latter accounts for a possible size reduction because of the compression,
whereas the former accounts for a possible subsequent size expansion
because of the alignment requirements.

The code still assumes that either underlying storage subsystems or
hardware is able to do read-modify-write when an L2ARC buffer size is
not a multiple of a disk's block size. This is true for 4KB sector disks
that provide 512B sector emulation, but may not be true in general.
In other words, we currently do not have any code to make sure that
an L2ARC buffer, whether compressed or not, which is used for physical I/O
has a suitable size.

Note that currently the cache device utilization is calculated based
on the physical size, not the allocated size. The same applies to l2_asize
kstat. That is wrong, but this commit does not fix that.
The accounting problem was introduced partially in
commit aad02571bc59671aa3103bb070ae365f531b0b62
and partially in 3038a2b421b40dc5ac11cd88423696618584f85a
(accounting became consistent but in favour of the wrong size).

FreeBSD and Linux.


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Description From Last Updated
Matthew Ahrens
Brian Behlendorf
Andriy Gapon
Andriy Gapon
Andriy Gapon
Saso Kiselkov
George Wilson
Steven Hartland
Justin Gibbs
Xin LI
Andriy Gapon
Review request changed

Status: Closed (submitted)