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[Qemu-block] [PULL 01/22] qcow2: Fix the calculation of the maximum L2 c
From: |
Kevin Wolf |
Subject: |
[Qemu-block] [PULL 01/22] qcow2: Fix the calculation of the maximum L2 cache size |
Date: |
Thu, 12 Sep 2019 15:45:43 +0200 |
From: Alberto Garcia <address@hidden>
The size of the qcow2 L2 cache defaults to 32 MB, which can be easily
larger than the maximum amount of L2 metadata that the image can have.
For example: with 64 KB clusters the user would need a qcow2 image
with a virtual size of 256 GB in order to have 32 MB of L2 metadata.
Because of that, since commit b749562d9822d14ef69c9eaa5f85903010b86c30
we forbid the L2 cache to become larger than the maximum amount of L2
metadata for the image, calculated using this formula:
uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
The problem with this formula is that the result should be rounded up
to the cluster size because an L2 table on disk always takes one full
cluster.
For example, a 1280 MB qcow2 image with 64 KB clusters needs exactly
160 KB of L2 metadata, but we need 192 KB on disk (3 clusters) even if
the last 32 KB of those are not going to be used.
However QEMU rounds the numbers down and only creates 2 cache tables
(128 KB), which is not enough for the image.
A quick test doing 4KB random writes on a 1280 MB image gives me
around 500 IOPS, while with the correct cache size I get 16K IOPS.
Cc: address@hidden
Signed-off-by: Alberto Garcia <address@hidden>
Signed-off-by: Kevin Wolf <address@hidden>
---
block/qcow2.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/block/qcow2.c b/block/qcow2.c
index 0882ff6e92..57734f20cf 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -828,7 +828,11 @@ static void read_cache_sizes(BlockDriverState *bs,
QemuOpts *opts,
bool l2_cache_entry_size_set;
int min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
- uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
+ uint64_t max_l2_entries = DIV_ROUND_UP(virtual_disk_size, s->cluster_size);
+ /* An L2 table is always one cluster in size so the max cache size
+ * should be a multiple of the cluster size. */
+ uint64_t max_l2_cache = ROUND_UP(max_l2_entries * sizeof(uint64_t),
+ s->cluster_size);
combined_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_CACHE_SIZE);
l2_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_SIZE);
--
2.20.1
- [Qemu-block] [PULL 00/22] Block layer patches, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 01/22] qcow2: Fix the calculation of the maximum L2 cache size,
Kevin Wolf <=
- [Qemu-block] [PULL 03/22] block/file-posix: Reduce xfsctl() use, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 02/22] job: drop job_drain, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 04/22] iotests: Test reverse sub-cluster qcow2 writes, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 05/22] pr-manager: Fix invalid g_free() crash bug, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 07/22] vpc: Return 0 from vpc_co_create() on success, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 06/22] file-posix: Fix has_write_zeroes after NO_FALLBACK, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 08/22] iotests: Add supported protocols to execute_test(), Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 10/22] iotests: Restrict nbd Python tests to nbd, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 09/22] iotests: Restrict file Python tests to file, Kevin Wolf, 2019/09/12
- [Qemu-block] [PULL 11/22] iotests: Test blockdev-create for vpc, Kevin Wolf, 2019/09/12