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Date: Mon, 4 Jan 2021 16:40:08 -0600 From: Eric Sandeen <sandeen@...deen.net> To: Theodore Ts'o <tytso@....edu>, Andres Freund <andres@...razel.de> Cc: linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org, linux-block@...r.kernel.org Subject: Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten extents? On 1/4/21 1:17 PM, Theodore Ts'o wrote: > On Tue, Dec 29, 2020 at 10:28:19PM -0800, Andres Freund wrote: >> >> 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 ... > > One thing to note is that there are some devices which support a write > zeros operation, but where it is *less* performant than actually > writing zeros via DMA'ing zero pages. Yes, that's insane. > Unfortunately, there are a insane devices out there.... > > This is not hypothetical; I know this because we tried using write > zeros in mke2fs, and I got regression complaints where > mke2fs/mkfs.ext4 got substantially slower for some devices. Was this "libext2fs: mkfs.ext3 really slow on centos 8.2" ? If so, wasn't the problem that it went from a few very large IOs to a multitude of per-block fallocate calls, a problem which was fixed by this commit? commit 86d6153417ddaccbe3d1f4466a374716006581f4 (HEAD) Author: Theodore Ts'o <tytso@....edu> Date: Sat Apr 25 11:41:24 2020 -0400 libext2fs: batch calls to ext2fs_zero_blocks2() When allocating blocks for an indirect block mapped file, accumulate blocks to be zero'ed and then call ext2fs_zero_blocks2() to zero them in large chunks instead of block by block. This significantly speeds up mkfs.ext3 since we don't send a large number of ZERO_RANGE requests to the kernel, and while the kernel does batch write requests, it is not batching ZERO_RANGE requests. It's more efficient to batch in userspace in any case, since it avoids unnecessary system calls. Reported-by: Mario Schuknecht <mario.schuknecht@...search-fe.de> Signed-off-by: Theodore Ts'o <tytso@....edu> or do I have the wrong report above? I ask because mkfs.xfs is now also using FALLOC_FL_ZERO_RANGE Thanks, -Eric
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