Richard W.M. Jones wrote:
> On Wed, Sep 03, 2008 at 11:19:04AM -0500, Eric Sandeen wrote:
>> I'd appreciate any and all testing, benchmarking & feedback that people
>> would be willing to do. Just getting more exposure in real-life
>> scenarios would be great.
>>
>> As with any filesystem, I wouldn't put your only copy of your most
>> precious data on it - use good sense about backups etc - but ext4 has
>> made good progress since F9 on both stability and performance, so have
>> at it!
>
> Persistent pre-allocation[1] is something that virt-manager could
> really use when it has to allocate multi-gigabyte images. A few
> questions about this feature though:
>
> (a) Is it exposed as a syscall anywhere? I don't see it in the header
> files of my Rawhide system (2.6.27).
Hm, I probably need to get the fallocate.h header file included if it's
not so that sys_fallocate can be used directly, but it is also exposed
via posix_fallocate in glibc - tested here on xfs just because xfs_bmap
is a handy way to show that it actually works via glibc:
[root@inode fallocate]# ./test_posix_fallocate testfile 0 16384
[root@inode fallocate]# xfs_bmap -vv testfile
testfile:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..31]: 138928..138959 0 (138928..138959) 32 10000
FLAG Values:
010000 Unwritten preallocated extent
001000 Doesn't begin on stripe unit
000100 Doesn't end on stripe unit
000010 Doesn't begin on stripe width
000001 Doesn't end on stripe width
the ->fallocate op is hooked up for ext4, xfs, and ocfs2 at this time.
> (b) Will preallocate "do the right thing" on filesystems that don't
> directly support it?
calling sys_fallocate() will give you -EOPNOTSUPP; using
posix_fallocate() falls back to writing zeros IIRC.
> (c) Does ext4 preallocate in the background? A synchronous
> preallocate call isn't much use to virt-manager.
It does not, but what is the concern? It doesn't take much time:
(on ext4 this time):
[root@inode test]# time test_posix_fallocate testfile 0 10737418240
real 0m0.009s
user 0m0.000s
sys 0m0.009s
[root@inode test]# ls -lh testfile
-rw-r--r-- 1 root root 10G 2008-09-03 12:30 testfile
[root@inode test]# du -hc testfile
11G testfile
11G total
-Eric
> Rich.
>
> [1] http://en.wikipedia.org/wiki/Ext4#Persistent_pre-allocation
>
#define _LARGEFILE64_SOURCE
#define _GNU_SOURCE
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
int main(int argc, char **argv)
{
int fd;
int ret;
loff_t offset;
loff_t len;
char *fname;
fname = argv[1];
offset = atoll(argv[2]);
len = atoll(argv[3]);
printf("file %s offset %llu (%s) length %llu (%s)\n", fname,
offset, argv[2], len, argv[3]);
fd = open(fname, O_CREAT|O_RDWR, 0666);
if (fd < 0) {
perror("Error opening file");
return 1;
}
ret = posix_fallocate64(fd, offset, len);
if (ret < 0)
perror("Error allocating space");
close(fd);
return 0;
}
--
fedora-devel-list mailing list
fedora-devel-list@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/fedora-devel-list
