From fe5cbc6e06c7d8b3a86f6f5491d74766bb5c2827 Mon Sep 17 00:00:00 2001
From: Markus Stockhausen <stockhausen@collogia.de>
Date: Mon, 15 Dec 2014 12:57:04 +1100
Subject: md/raid6 algorithms: delta syndrome functions
v3: s-o-b comment, explanation of performance and descision for
the start/stop implementation
Implementing rmw functionality for RAID6 requires optimized syndrome
calculation. Up to now we can only generate a complete syndrome. The
target P/Q pages are always overwritten. With this patch we provide
a framework for inplace P/Q modification. In the first place simply
fill those functions with NULL values.
xor_syndrome() has two additional parameters: start & stop. These
will indicate the first and last page that are changing during a
rmw run. That makes it possible to avoid several unneccessary loops
and speed up calculation. The caller needs to implement the following
logic to make the functions work.
1) xor_syndrome(disks, start, stop, ...): "Remove" all data of source
blocks inside P/Q between (and including) start and end.
2) modify any block with start <= block <= stop
3) xor_syndrome(disks, start, stop, ...): "Reinsert" all data of
source blocks into P/Q between (and including) start and end.
Pages between start and stop that won't be changed should be filled
with a pointer to the kernel zero page. The reasons for not taking NULL
pages are:
1) Algorithms cross the whole source data line by line. Thus avoid
additional branches.
2) Having a NULL page avoids calculating the XOR P parity but still
need calulation steps for the Q parity. Depending on the algorithm
unrolling that might be only a difference of 2 instructions per loop.
The benchmark numbers of the gen_syndrome() functions are displayed in
the kernel log. Do the same for the xor_syndrome() functions. This
will help to analyze performance problems and give an rough estimate
how well the algorithm works. The choice of the fastest algorithm will
still depend on the gen_syndrome() performance.
With the start/stop page implementation the speed can vary a lot in real
life. E.g. a change of page 0 & page 15 on a stripe will be harder to
compute than the case where page 0 & page 1 are XOR candidates. To be not
to enthusiatic about the expected speeds we will run a worse case test
that simulates a change on the upper half of the stripe. So we do:
1) calculation of P/Q for the upper pages
2) continuation of Q for the lower (empty) pages
Signed-off-by: Markus Stockhausen <stockhausen@collogia.de>
Signed-off-by: NeilBrown <neilb@suse.de>
---
include/linux/raid/pq.h | 1 +
1 file changed, 1 insertion(+)
(limited to 'include/linux')
diff --git a/include/linux/raid/pq.h b/include/linux/raid/pq.h
index 73069cb6c54a..a7a06d1dcf9c 100644
--- a/include/linux/raid/pq.h
+++ b/include/linux/raid/pq.h
@@ -72,6 +72,7 @@ extern const char raid6_empty_zero_page[PAGE_SIZE];
/* Routine choices */
struct raid6_calls {
void (*gen_syndrome)(int, size_t, void **);
+ void (*xor_syndrome)(int, int, int, size_t, void **);
int (*valid)(void); /* Returns 1 if this routine set is usable */
const char *name; /* Name of this routine set */
int prefer; /* Has special performance attribute */
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