Episode Details

New ZFS features landing in FreeBSD, MAP_STACK for OpenBSD, how to write safer C code with Clang’s address sanitizer, Michael W. Lucas on sponsor gifts, TCP blackbox recorder, and Dell disk system hacking.

Headlines

[A number of Upstream ZFS features landed in FreeBSD this week]

• 9188 increase size of dbuf cache to reduce indirect block decompression

  With compressed ARC (6950) we use up to 25% of our CPU to decompress indirect blocks, under a workload of random cached reads. To reduce this decompression cost, we would like to increase the size of the dbuf cache so that more indirect blocks can be stored uncompressed. If we are caching entire large files of recordsize=8K, the indirect blocks use 1/64th as much memory as the data blocks (assuming they have the same compression ratio). We suggest making the dbuf cache be 1/32nd of all memory, so that in this scenario we should be able to keep all the indirect blocks decompressed in the dbuf cache. (We want it to be more than the 1/64th that the indirect blocks would use because we need to cache other stuff in the dbuf cache as well.) In real world workloads, this won't help as dramatically as the example above, but we think it's still worth it because the risk of decreasing performance is low. The potential negative performance impact is that we will be slightly reducing the size of the ARC (by ~3%).

• 9166 zfs storage pool checkpoint

  The idea of Storage Pool Checkpoint (aka zpool checkpoint) deals with exactly that. It can be thought of as a “pool-wide snapshot” (or a variation of extreme rewind that doesn’t corrupt your data). It remembers the entire state of the pool at the point that it was taken and the user can revert back to it later or discard it. Its generic use case is an administrator that is about to perform a set of destructive actions to ZFS as part of a critical procedure. She takes a checkpoint of the pool before performing the actions, then rewinds back to it if one of them fails or puts the pool into an unexpected state. Otherwise, she discards it. With the assumption that no one else is making modifications to ZFS, she basically wraps all these actions into a “high-level transaction”.

• More information

• 8484 Implement aggregate sum and use for arc counters

  In pursuit of improving performance on multi-core systems, we should implements fanned out counters and use them to improve the performance of some of the arc statistics. These stats are updated extremely frequently, and can consume a significant amount of CPU time.

• And a small bug fix authored by me:

• 9321 arcloancompressedbuf() can increment arcloanedbytes by the wrong value

  arcloancompressedbuf() increments arcloanedbytes by psize unconditionally In the case of zfscompressedarcenabled=0, when the buf is returned via arcreturnbuf(), if ARCBUFCOMPRESSED(buf) is false, then arcloanedbytes is decremented by lsize, not psize. Switch to using arcbufsize(buf), instead of psize, which will return psize or lsize, depending on the result of ARCBUF_COMPRESSED(buf).

MAP_STACK for OpenBSD

Almost 2 decades ago we started work on W^X. The concept was simple. Pages that are writable, should not