dm cache: emit a warning message if there are a lot of cache blocks
Loading and saving millions of block mappings takes time. We may as
well explain what's going on, and encourage people to use a larger
cache block size.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 6e36a07..abdd45d 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -2301,6 +2301,19 @@
return discard_block_size;
}
+static void set_cache_size(struct cache *cache, dm_cblock_t size)
+{
+ dm_block_t nr_blocks = from_cblock(size);
+
+ if (nr_blocks > (1 << 20) && cache->cache_size != size)
+ DMWARN_LIMIT("You have created a cache device with a lot of individual cache blocks (%llu)\n"
+ "All these mappings can consume a lot of kernel memory, and take some time to read/write.\n"
+ "Please consider increasing the cache block size to reduce the overall cache block count.",
+ (unsigned long long) nr_blocks);
+
+ cache->cache_size = size;
+}
+
#define DEFAULT_MIGRATION_THRESHOLD 2048
static int cache_create(struct cache_args *ca, struct cache **result)
@@ -2356,10 +2369,10 @@
cache->sectors_per_block_shift = -1;
cache_size = block_div(cache_size, ca->block_size);
- cache->cache_size = to_cblock(cache_size);
+ set_cache_size(cache, to_cblock(cache_size));
} else {
cache->sectors_per_block_shift = __ffs(ca->block_size);
- cache->cache_size = to_cblock(ca->cache_sectors >> cache->sectors_per_block_shift);
+ set_cache_size(cache, to_cblock(ca->cache_sectors >> cache->sectors_per_block_shift));
}
r = create_cache_policy(cache, ca, error);
@@ -2856,7 +2869,7 @@
return r;
}
- cache->cache_size = new_size;
+ set_cache_size(cache, new_size);
return 0;
}