Messages in this thread Patch in this message |  | | | Date | Mon, 22 Feb 2010 15:49:20 -0500 | | From | Mike Snitzer <> | | Subject | [RFC PATCH] block: warn if blk_stack_limits() undermines atomicity |
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Linux Device Mapper (DM) and Software Raid (MD) device drivers can be used to arbitrarily combine devices with different "I/O Limits". The kernel's block layer goes to great lengths to reasonably combine the "I/O Limits" of the individual devices. The kernel will not prevent combining heterogenuous devices but the user should be aware of the risk associated with doing so.
For instance, a 512 byte device and a 4K device may be combined into a single logical DM device; the resulting DM device would have a logical_block_size of 4K. Filesystems layered on such a hybrid device assume that 4K will be written atomically but in reality that 4K will be split into 8 512 byte IOs when issued to the 512 byte device. Using a 4K logical_block_size for the higher-level DM device increases potential for a partial write to the 512b device if there is a system crash.
If combining multiple devices' "I/O Limits" results in a conflict the block layer will report a warning that the device is more susceptible to partial writes and misaligned. [NOTE: setting "misaligned" for this warning is somewhat awkward but blk_stack_limits() return of -1 can be viewed as there was an "alignment inconsistency". Would it be better to return -1 but avoid setting t->misaligned?]
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/block/blk-settings.c b/block/blk-settings.c index 5eeb9e0..33bebe7 100644 --- a/block/blk-settings.c +++ b/block/blk-settings.c @@ -566,8 +566,16 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b, } } + top = t->logical_block_size; t->logical_block_size = max(t->logical_block_size, b->logical_block_size); + if (top && top < t->logical_block_size) { + printk(KERN_NOTICE "Warning: changing logical_block_size of top device " + "(from %u to %u) increases potential for partial writes\n", + top, t->logical_block_size); + t->misaligned = 1; + ret = -1; + } t->physical_block_size = max(t->physical_block_size, b->physical_block_size);
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