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copy-on-read: skip non-guest reads if no copy needed

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If the flag BDRV_REQ_PREFETCH was set, skip idling read/write
operations in COR-driver. It can be taken into account for the
COR-algorithms optimization. That check is being made during the
block stream job by the moment.

Add the BDRV_REQ_PREFETCH flag to the supported_read_flags of the
COR-filter.

block: Modify the comment for the flag BDRV_REQ_PREFETCH as we are
going to use it alone and pass it to the COR-filter driver for further
processing.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201216061703.70908-9-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Andrey Shinkevich 2020-12-16 09:16:58 +03:00 committed by Max Reitz
parent 897dd0ec4f
commit e275458b29
2 changed files with 15 additions and 7 deletions
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14
block/copy-on-read.c
View file

@ -50,6 +50,8 @@ static int cor_open(BlockDriverState *bs, QDict *options, int flags,
return -EINVAL;
}
bs->supported_read_flags = BDRV_REQ_PREFETCH;
bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
(BDRV_REQ_FUA & bs->file->bs->supported_write_flags);
@ -172,10 +174,14 @@ static int coroutine_fn cor_co_preadv_part(BlockDriverState *bs,
}
}
ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
local_flags);
if (ret < 0) {
return ret;
/* Skip if neither read nor write are needed */
if ((local_flags & (BDRV_REQ_PREFETCH | BDRV_REQ_COPY_ON_READ)) !=
BDRV_REQ_PREFETCH) {
ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
local_flags);
if (ret < 0) {
return ret;
}
}
offset += n;

8
include/block/block.h
View file

@ -72,9 +72,11 @@ typedef enum {
BDRV_REQ_NO_FALLBACK = 0x100,
/*
* BDRV_REQ_PREFETCH may be used only together with BDRV_REQ_COPY_ON_READ
* on read request and means that caller doesn't really need data to be
* written to qiov parameter which may be NULL.
* BDRV_REQ_PREFETCH makes sense only in the context of copy-on-read
* (i.e., together with the BDRV_REQ_COPY_ON_READ flag or when a COR
* filter is involved), in which case it signals that the COR operation
* need not read the data into memory (qiov) but only ensure they are
* copied to the top layer (i.e., that COR operation is done).
*/
BDRV_REQ_PREFETCH = 0x200,