Re: How to flush data to disk reliably?
From: Grzegorz Kulewski
Date: Mon May 02 2005 - 14:19:53 EST
Thanks for your fast response!
On Mon, 2 May 2005, Alan Cox wrote:
I am asking how to flush the data from these logs to disk. I know of
several methods:
1. open with O_SYNC,
2. sync(2),
3. fsync,
4. fdatasync,
5. msync (if they are mmaped).
Which of these are best and most reliable for (a/b) and for (IDE/SCSI)?
For scsi the combination of O_SYNC and ext3 or fsync and ext3 should be
reliable. fdatasync doesn't write back all the metadata so depending on
the use may not be sufficient. FAT based fs's I believe you need a
current kernel for full O_SYNC behaviour.
What about other filesystems? Does anybody know anwser for Reiserfs3,
Reiser4, JFS, XFS and any other popular server filesystems? I assume that
if log file is some block device (like partition) both O_SYNC and fsync
will work? What about ext2? What about some strange RAID/DM/NBD
configurations? (I do not know in advance what our customers will use so I
need portable method.)
What are differences between them?
See the manual pages and/or standard.
I already saw them. But I am asking about current implementation status on
Linux. For example does fsync differ from fdatasync if file is block
device? Does O_SYNC always equal "write; fsync" for all not read only
operations? Is it faster because only one syscall is executed?
Also flushing should be slow (for example 50 flushes/s) because disk seeks
are slow. So if I need say 200 reliable writes to log per second may I put
4 independent disks into the server and use them as 4 independent log
files? But fsync operation blocks. Is there any "submit flush request and
get some info when done" command or should I use 4 threads / processes?
Thanks in advance,
Grzegorz Kulewski
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