scarlet-storm
6a389701b2
Ensure we don't create a loop device on top of a physical block device. This leads to huge performance degradation of discard operations if the physical device does not support discard_on_zeroes. - loop device historical semantics dictates that when the device is discarded, it needs to return zero data on read. This can be implemented easily on a filesystem. since fallocate zero-range would return immediately & the holes are handled at the filesystem level to return zero data on read. - For a raw block device, the feature (discard_zeroes_data) depends on the capabilities of the physical device that is exposed to the block layer by the driver. This means that to guarantee that the loop device stacked on a block device returns zero on discarded data, it needs to convert discarded range into write_zero op on the block device. https://github.com/torvalds/linux/blob/63676eefb7a026d04b51dcb7aaf54f358517a2ec/drivers/block/loop.c#L773 For example on one of my local nvme I can see the following: cat /sys/class/block/nvme1n1/queue/write_zeroes_max_bytes 131072 cat /sys/class/block/nvme0n1/queue/discard_max_hw_bytes 2199023255040 This means maximum size of a write_zero operation can be 128KiB & maximum size of discard operation can be 2TiB on the block device. So discarding for example 1TB of data, which would be a single block device operation, gets split into 8.3 million block device operations when issued on top of stacked loop device.
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