On Tue, Dec 29, 2020 at 10:28:19PM -0800, Andres Freund wrote:

Hi,

For things like database journals using fallocate(0) is not sufficient,
as writing into the the pre-allocated data with O_DIRECT | O_DSYNC
writes requires the unwritten extents to be converted, which in turn
requires journal operations.

The performance difference in a journalling workload (lots of
sequential, low-iodepth, often small, writes) is quite remarkable. Even
on quite fast devices:

    andres@awork3:/mnt/t3$ grep /mnt/t3 /proc/mounts
    /dev/nvme1n1 /mnt/t3 xfs rw,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0

    andres@awork3:/mnt/t3$ fallocate -l $((1024*1024*1024)) test_file

    andres@awork3:/mnt/t3$ dd if=/dev/zero of=test_file bs=4096 conv=notrunc iflag=count_bytes count=$((1024*1024*1024)) oflag=direct,dsync
    262144+0 records in
    262144+0 records out
    1073741824 bytes (1.1 GB, 1.0 GiB) copied, 117.587 s, 9.1 MB/s

    andres@awork3:/mnt/t3$ dd if=/dev/zero of=test_file bs=4096 conv=notrunc iflag=count_bytes count=$((1024*1024*1024)) oflag=direct,dsync
    262144+0 records in
    262144+0 records out
    1073741824 bytes (1.1 GB, 1.0 GiB) copied, 3.69125 s, 291 MB/s

    andres@awork3:/mnt/t3$ fallocate -z -l $((1024*1024*1024)) test_file

    andres@awork3:/mnt/t3$ dd if=/dev/zero of=test_file bs=4096 conv=notrunc iflag=count_bytes count=$((1024*1024*1024)) oflag=direct,dsync
    z262144+0 records in
    262144+0 records out
    1073741824 bytes (1.1 GB, 1.0 GiB) copied, 109.398 s, 9.8 MB/s

    andres@awork3:/mnt/t3$ dd if=/dev/zero of=test_file bs=4096 conv=notrunc iflag=count_bytes count=$((1024*1024*1024)) oflag=direct,dsync
    262144+0 records in
    262144+0 records out
    1073741824 bytes (1.1 GB, 1.0 GiB) copied, 3.76166 s, 285 MB/s


The way around that, from a database's perspective, is obviously to just
overwrite the file "manually" after fallocate()ing it, utilizing larger
writes, and then to recycle the file.


But that's a fair bit of unnecessary IO from userspace, and it's IO that
the kernel can do more efficiently on a number of types of block
devices, e.g. by utilizing write-zeroes.


Which brings me to $subject:

Would it make sense to add a variant of FALLOC_FL_ZERO_RANGE that
doesn't convert extents into unwritten extents, but instead uses
blkdev_issue_zeroout() if supported?  Mostly interested in xfs/ext4
myself, but ...

Doing so as a variant of FALLOC_FL_ZERO_RANGE seems to make the most
sense, as that'd work reasonably efficiently to initialize newly
allocated space as well as for zeroing out previously used file space.


As blkdev_issue_zeroout() already has a fallback path it seems this
should be doable without too much concern for which devices have write
zeroes, and which do not?

Question: do you want the kernel to write zeroes even for devices that
don't support accelerated zeroing?  Since I assume that if the fallocate
fails you'll fall back to writing zeroes from userspace anyway...

Second question: Would it help to have a FALLOC_FL_DRY_RUN flag that
could be used to probe if a file supports fallocate without actually
changing anything?  I'm (separately) pursuing a fix for the loop device
not being able to figure out if a file actually supports a particular
fallocate mode.

--D

Greetings,

Andres Freund