zfsonlinux/zfs — pull request #858 by dechamps (2012-07-31 09:00:34+00:00)
Title: Limit the number of blocks to discard at once

The number of blocks that can be discarded in one `BLKDISCARD` ioctl on a zvol is currently unlimited. Some applications, such as mkfs, discard the whole volume at once and they use the maximum possible discard size
to do that. As a result, several gigabytes discard requests are not uncommon.

Unfortunately, if a large amount of data is allocated in the zvol, ZFS can be quite slow to process discard requests. This is especially true if the volblocksize is low (e.g. the 8K default). As a result, very large discard requests can take a very long time (seconds to minutes under heavy load) to complete. This can cause a number of problems, most notably if the zvol is accessed remotely (e.g. via iSCSI), in which case the client has a high probability of timing out on the request.

This patch solves the issue by adding a new tunable module parameter: `zvol_max_discard_blocks`. This indicates the maximum possible range, in zvol blocks, of one discard operation. It is set by default to 16384
blocks, which appears to be a good tradeoff. Using the default volblocksize of 8K this is equivalent to 128 MB. When using the maximum volblocksize of 128K this is equivalent to 2 GB.