Re: [PATCH 2/2] block: file-posix: Replace posix_fallocate with fallocate
On Mon, Sep 14, 2020 at 8:32 PM Daniel P. Berrangé wrote: > > On Mon, Aug 31, 2020 at 05:01:27PM +0300, Nir Soffer wrote: > > If fallocate() is not supported, posix_fallocate() falls back to > > inefficient allocation, writing one byte for every 4k bytes[1]. This is > > very slow compared with writing zeros. In oVirt we measured ~400% > > improvement in allocation time when replacing posix_fallocate() with > > manually writing zeroes[2]. > > > > We also know that posix_fallocated() does not work well when using OFD > > locks[3]. We don't know the reason yet for this issue yet. > > > > Change preallocate_falloc() to use fallocate() instead of > > posix_falloate(), and fall back to full preallocation if not supported. > > > > Here are quick test results with this change. > > > > Before (qemu-img-5.1.0-2.fc32.x86_64): > > > > $ time qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw 6g > > Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 > > preallocation=falloc > > > > real 0m42.100s > > user 0m0.602s > > sys 0m4.137s > > > > NFS stats: > > calls retransauthrefrshwrite > > 15715830 1572205 1571321 > > > > After: > > > > $ time ./qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw > > 6g > > Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 > > preallocation=falloc > > > > real 0m15.551s > > user 0m0.070s > > sys 0m2.623s > > > > NFS stats: > > calls retransauthrefrshwrite > > 24620 0 24624 24567 > > > > [1] > > https://code.woboq.org/userspace/glibc/sysdeps/posix/posix_fallocate.c.html#96 > > [2] https://bugzilla.redhat.com/1850267#c25 > > [3] https://bugzilla.redhat.com/1851097 > > This bug appears to be private to RH employees only, so rather than link > to it, please summarize any important facts in it for benefit of nonn-RH > QEMU contributors. Thanks, I missed that detail when linking to the bug. The bug is public now. > > Signed-off-by: Nir Soffer > > --- > > block/file-posix.c | 32 +- > > docs/system/qemu-block-drivers.rst.inc | 11 + > > docs/tools/qemu-img.rst| 11 + > > qapi/block-core.json | 4 ++-- > > 4 files changed, 25 insertions(+), 33 deletions(-) > > Regards, > Daniel > -- > |: https://berrange.com -o-https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o-https://fstop138.berrange.com :| > |: https://entangle-photo.org-o-https://www.instagram.com/dberrange :| >
Re: [PATCH 2/2] block: file-posix: Replace posix_fallocate with fallocate
On Mon, Aug 31, 2020 at 05:01:27PM +0300, Nir Soffer wrote: > If fallocate() is not supported, posix_fallocate() falls back to > inefficient allocation, writing one byte for every 4k bytes[1]. This is > very slow compared with writing zeros. In oVirt we measured ~400% > improvement in allocation time when replacing posix_fallocate() with > manually writing zeroes[2]. > > We also know that posix_fallocated() does not work well when using OFD > locks[3]. We don't know the reason yet for this issue yet. > > Change preallocate_falloc() to use fallocate() instead of > posix_falloate(), and fall back to full preallocation if not supported. > > Here are quick test results with this change. > > Before (qemu-img-5.1.0-2.fc32.x86_64): > > $ time qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw 6g > Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 preallocation=falloc > > real 0m42.100s > user 0m0.602s > sys 0m4.137s > > NFS stats: > calls retrans authrefrshwrite > 1571583 0 1572205 1571321 > > After: > > $ time ./qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw 6g > Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 preallocation=falloc > > real 0m15.551s > user 0m0.070s > sys 0m2.623s > > NFS stats: > calls retrans authrefrshwrite > 24620 0 24624 24567 > > [1] > https://code.woboq.org/userspace/glibc/sysdeps/posix/posix_fallocate.c.html#96 > [2] https://bugzilla.redhat.com/1850267#c25 > [3] https://bugzilla.redhat.com/1851097 This bug appears to be private to RH employees only, so rather than link to it, please summarize any important facts in it for benefit of nonn-RH QEMU contributors. > > Signed-off-by: Nir Soffer > --- > block/file-posix.c | 32 +- > docs/system/qemu-block-drivers.rst.inc | 11 + > docs/tools/qemu-img.rst| 11 + > qapi/block-core.json | 4 ++-- > 4 files changed, 25 insertions(+), 33 deletions(-) Regards, Daniel -- |: https://berrange.com -o-https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o-https://fstop138.berrange.com :| |: https://entangle-photo.org-o-https://www.instagram.com/dberrange :|
Re: [PATCH 2/2] block: file-posix: Replace posix_fallocate with fallocate
On Mon 31 Aug 2020 04:01:27 PM CEST, Nir Soffer wrote: > If fallocate() is not supported, posix_fallocate() falls back to > inefficient allocation, writing one byte for every 4k bytes[1]. This is > very slow compared with writing zeros. In oVirt we measured ~400% > improvement in allocation time when replacing posix_fallocate() with > manually writing zeroes[2]. > > We also know that posix_fallocated() does not work well when using OFD > locks[3]. We don't know the reason yet for this issue yet. > > Change preallocate_falloc() to use fallocate() instead of > posix_falloate(), and fall back to full preallocation if not > supported. > case PREALLOC_MODE_FALLOC: > result = preallocate_falloc(fd, current_length, offset, errp); > -goto out; > +if (result != -ENOTSUP) > +goto out; > +/* If fallocate() is not supported, fallback to full preallocation. > */ With the missing braces in this if statement, Reviewed-by: Alberto Garcia Berto
[PATCH 2/2] block: file-posix: Replace posix_fallocate with fallocate
If fallocate() is not supported, posix_fallocate() falls back to
inefficient allocation, writing one byte for every 4k bytes[1]. This is
very slow compared with writing zeros. In oVirt we measured ~400%
improvement in allocation time when replacing posix_fallocate() with
manually writing zeroes[2].
We also know that posix_fallocated() does not work well when using OFD
locks[3]. We don't know the reason yet for this issue yet.
Change preallocate_falloc() to use fallocate() instead of
posix_falloate(), and fall back to full preallocation if not supported.
Here are quick test results with this change.
Before (qemu-img-5.1.0-2.fc32.x86_64):
$ time qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw 6g
Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 preallocation=falloc
real 0m42.100s
user 0m0.602s
sys 0m4.137s
NFS stats:
calls retrans authrefrshwrite
1571583 0 1572205 1571321
After:
$ time ./qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw 6g
Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 preallocation=falloc
real 0m15.551s
user 0m0.070s
sys 0m2.623s
NFS stats:
calls retrans authrefrshwrite
24620 0 24624 24567
[1]
https://code.woboq.org/userspace/glibc/sysdeps/posix/posix_fallocate.c.html#96
[2] https://bugzilla.redhat.com/1850267#c25
[3] https://bugzilla.redhat.com/1851097
Signed-off-by: Nir Soffer
---
block/file-posix.c | 32 +-
docs/system/qemu-block-drivers.rst.inc | 11 +
docs/tools/qemu-img.rst| 11 +
qapi/block-core.json | 4 ++--
4 files changed, 25 insertions(+), 33 deletions(-)
diff --git a/block/file-posix.c b/block/file-posix.c
index 341ffb1cb4..eac3c0b412 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1835,36 +1835,24 @@ static int allocate_first_block(int fd, size_t max_size)
static int preallocate_falloc(int fd, int64_t current_length, int64_t offset,
Error **errp)
{
-#ifdef CONFIG_POSIX_FALLOCATE
+#ifdef CONFIG_FALLOCATE
int result;
if (offset == current_length)
return 0;
-/*
- * Truncating before posix_fallocate() makes it about twice slower on
- * file systems that do not support fallocate(), trying to check if a
- * block is allocated before allocating it, so don't do that here.
- */
-
-result = -posix_fallocate(fd, current_length,
- offset - current_length);
+result = do_fallocate(fd, 0, current_length, offset - current_length);
if (result != 0) {
-/* posix_fallocate() doesn't set errno. */
-error_setg_errno(errp, -result,
- "Could not preallocate new data");
+error_setg_errno(errp, -result, "Could not preallocate new data");
return result;
}
if (current_length == 0) {
/*
- * posix_fallocate() uses fallocate() if the filesystem supports
- * it, or fallback to manually writing zeroes. If fallocate()
- * was used, unaligned reads from the fallocated area in
- * raw_probe_alignment() will succeed, hence we need to allocate
- * the first block.
+ * Unaligned reads from the fallocated area in raw_probe_alignment()
+ * will succeed, hence we need to allocate the first block.
*
- * Optimize future alignment probing; ignore failures.
+ * Optimizes future alignment probing; ignore failures.
*/
allocate_first_block(fd, offset);
}
@@ -1973,10 +1961,12 @@ static int handle_aiocb_truncate(void *opaque)
}
switch (prealloc) {
-#ifdef CONFIG_POSIX_FALLOCATE
+#ifdef CONFIG_FALLOCATE
case PREALLOC_MODE_FALLOC:
result = preallocate_falloc(fd, current_length, offset, errp);
-goto out;
+if (result != -ENOTSUP)
+goto out;
+/* If fallocate() is not supported, fallback to full preallocation. */
#endif
case PREALLOC_MODE_FULL:
result = preallocate_full(fd, current_length, offset, errp);
@@ -3080,7 +3070,7 @@ static QemuOptsList raw_create_opts = {
.name = BLOCK_OPT_PREALLOC,
.type = QEMU_OPT_STRING,
.help = "Preallocation mode (allowed values: off"
-#ifdef CONFIG_POSIX_FALLOCATE
+#ifdef CONFIG_FALLOCATE
", falloc"
#endif
", full)"
diff --git a/docs/system/qemu-block-drivers.rst.inc
b/docs/system/qemu-block-drivers.rst.inc
index b052a6d14e..8e4acf397e 100644
--- a/docs/system/qemu-block-drivers.rst.inc
+++ b/docs/system/qemu-block-drivers.rst.inc
@@ -25,11 +25,12 @@ This section describes each format and the options that are
supported for it.
.. program:: raw
.. option:: preallocation
-Preallocation mode (allowed values: ``off``, ``falloc``,
-``full``). ``falloc`` mode preallocates space for image by
-
