Storage Performance (Status and improvements)

[Wescoeur]

This is a a non-exhaustive list of ideas and potential improvements to test concerning the storage layer:

• Indirect segments are not supported by blktap (see: [RFE] Support indirect descriptors feature xapi-project/blktap#287), is it a good idea to implement this feature in tapdisk and/or qemu-dp?
• qemu-dp uses blkif in the same way as tapdisk but we have a better performance with tapdisk. Why? I think there is a specific bottleneck in the qemu-dp process (RW API maybe).
• Modify the ring-page-order param in blkif.
  => No better performance with NVMe (see: https://xcp-ng.org/forum/topic/1596/solved-slow-disk-write-speed-inside-vm/21).
  => Update: The param is now available on the master branch of tapdisk. (We must test it!)
  => Update2: The tapdisk/module param doesn't support a value of 4 for the moment.
• Test qemu-dp with io_uring support (see: https://patchew.org/QEMU/20191025160444.31632-1-stefanha@redhat.com/).
• Try to use persistant grant (with LRU cache or LIFO queue) + try to build a similar map/unmap mechanism with deferred unmap. Is it really true that map/unmap is always slow compared to buffer copy between guest and host?
• Use two rings: one for the requests and another for responses (bad usage of the cache line).
• Avoid context switches, IRQs (polling is good when there are a lot of data)... A solution to test is to use a process with N threads on N physical cores. Each process thread must map a VM thread (1:1 map) which executes requests and responses (see: https://github.com/torvalds/linux/blob/master/include/xen/interface/io/blkif.h#L32)
• Test the differents Linux I/O schedulers at different levels.
• Reproduce ext4 EIO problem with io_uring and O_DIRECT.
• Use ASan in xcp-ng-async-io because Valgrind cannot be used for the moment.
• Make xcp-ng-async-io RPM + update blktap RPM.
• Try to use a more recent version of qemu-dp.
• Try to patch qemu-dp with xcp-ng-async-io.
• Bench with XFS! (When io_uring will be stable, i.e. no EIO errors with ext4.)

[maxcuttins]

Uhuuu.. this is something I would like to follow ❤️

[Wescoeur]

Pooling REQs: Benchs with tapdisk, NVMe disk and a AMD Ryzen 7 2700

Idea: Bench performance with no-pooling, full-pooling or adaptive pooling (See: https://github.com/xapi-project/blktap/blob/d2f49df6580a891a1317398b9e63f87ea0189571/drivers/td-blkif.c#L352)

Results

Analysis

When pooling is always enabled, the cpu usage is around 100% (seems logic) and we don't have a significant gain, without polling the random write is negatively impacted. So the default process is the best.

Concerning qemu-dp we can try to use a similar adaptive pooling to increase (a little) the random write performance. I'm not sure if this mechanism is currently implemented but for the moment it's not the most important thing to investigate.