Re: Ext4: batched discard support

[Greg Freemyer <greg.freemyer@gmail.com>]

Adding Mark Lord in cc.

He wrote a preliminary discard solution last summer.  I'm not sure how
it has progressed.

Mark, you can find the 2 patches at:

http://patchwork.ozlabs.org/patch/50441/
http://patchwork.ozlabs.org/patch/50442/

Greg

On Mon, Apr 19, 2010 at 6:55 AM, Lukas Czerner <lczerner@redhat.com> wrote:
> Hi all,
>
> I would like to present a new way to deal with TRIM in ext4 file system.
> The current solution is not ideal because of its bad performance impact.
> So basic idea to improve things is to avoid discarding every time some
> blocks are freed. and instead batching is together into bigger trims,
> which tends to be more effective.
>
> The basic idea behind my discard support is to create an ioctl which
> walks through all the free extents in each allocating group and discard
> those extents. As an addition to improve its performance one can specify
> minimum free extent length, so ioctl will not bother with shorter extents.
>
> This of course means, that with each invocation the ioctl must walk
> through whole file system, checking and discarding free extents, which
> is not very efficient. The best way to avoid this is to keep track of
> deleted (freed) blocks. Then the ioctl have to trim just those free
> extents which were recently freed.
>
> In order to implement this I have added new bitmap into ext4_group_info
> (bb_bitmap_deleted) which stores recently freed blocks. The ioctl then
> walk through bb_bitmap_deleted, compare deleted extents with free
> extents trim them and then removes it from the bb_bitmap_deleted.
>
> But you may notice, that there is one problem. bb_bitmap_deleted does
> not survive umount. To bypass the problem the first ioctl call have to
> walk through whole file system trimming all free extents. But there is a
> better solution to this problem. The bb_bitmap_deleted can be stored on
> disk an can be restored in mount time along with other bitmaps, but I
> think it is a quite big change and should be discussed further.
>
> I have also benchmarked it a little. You can find results here:
>
> people.redhat.com/jmoyer/discard/ext4_batched_discard/
>
> comparison with current solution included. Keep in mind that ideal ioctl
> invocation interval is yet to be determined, so in benchmark I have used
> the performance-worst scenario - without any sleep between execution.
>
>
> There are two patches for this. The first one just creates file system
> independent ioctl for this and the second one it the batched discard
> support itself.
>
> I will very much appreciate any comment on this, your opinions, ideas to
> make this better etc. Thanks.
>
> If you want to try it, just create EXT4 file system mount it and invoke
> ioctl on the mount point. You can use following code for this (I have
> taken this from xfs patch for the same thing). You can also see some
> debugging messages, but you may want to set EXT4FS_DEBUG for this.
>
> #include <errno.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdint.h>
> #include <sys/ioctl.h>
>
> #define FITRIM          _IOWR('X', 121, int)
>
> int main(int argc, char **argv)
> {
>        int minsize = 4096;
>        int fd;
>
>        if (argc != 2) {
>                fprintf(stderr, "usage: %s mountpoint\n", argv[0]);
>                return 1;
>        }
>
>        fd = open(argv[1], O_RDONLY);
>        if (fd < 0) {
>                perror("open");
>                return 1;
>        }
>
>        if (ioctl(fd, FITRIM, &minsize)) {
>                if (errno == EOPNOTSUPP)
>                        fprintf(stderr, "TRIM not supported\n");
>                else
>                        perror("EXT4_IOC_TRIM");
>                return 1;
>        }
>
>        return 0;
> }
>
>  fs/ioctl.c         |   31 +++++++++++++++++++++++++++++++
>  include/linux/fs.h |    2 ++
>  2 files changed, 33 insertions(+), 0 deletions(-)
>
>  fs/ext4/ext4.h    |    4 +
>  fs/ext4/mballoc.c |  207 ++++++++++++++++++++++++++++++++++++++++++++++++++---
>  fs/ext4/super.c   |    1 +
>  3 files changed, 202 insertions(+), 10 deletions(-)
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