dan wrote:
>
> On Wed, Sep 16, 2009 at 3:24 AM, Tino Schwarze <backuppc.lists AT tisc DOT de
> <mailto:backuppc.lists AT tisc DOT de>> wrote:
>
> On Tue, Sep 15, 2009 at 03:12:28PM -0800, Chris Robertson wrote:
>
> > In short, it works for me.
>
> [...]
>
> Wow, thanks for sharing your experience. I figure that DRBD is a nice
> way to RAID-1 across multiple hosts for failover purposes. I didn't
> expect it to perform that well - I'll look into it for Samba/NFS
> backup
> server... (It nicely integrates with hearbeat BTW)
>
>
> drdb (or AoE or iSCSI really) are great over local networks. latency
> is usually pretty low and they are all low-overhead.
>
> reading this post, he is just putting the xfs journal on drdb.
Read it again. :o) Both the external XFS journal (logdev=/dev/drbd1)
AND the data partition (/dev/drbd0) are DRBD mirrored. It would be
silly to have only one or the other saved in a DR scenario.
> All this really does is save the disk heads for doing some seeks
> which will lower the average latency of the drive somewhat because
> there is less travel(some sites online say up to 30% write improvement
> but that was surely a special workload). You could do this to a local
> drive also. Since this is just the journal, the only concern here is
> having the system go down and losing the journal device at the same time.
>
> FYI, you can put the journal on a flash key as well as the journal
> doesnt take that much space.
I would not recommend doing this. My journal has seen 229GB of writes
since I started using DRBD (about a month) but only 260MB of reads. So
while my journal is small enough (128MB*) to fit on pretty much any USB
key, it is written to extensively, and read from seldomly. Not a very
good usage situation for flash.
> the journal gets overwritten once the filesystem confirms the write
> so you really only have to store as much data as the filesystem has
> queue up to write. really a 1GB journal is way overkill but a 1GB
> flash drive is cheap. Also, the journal doesnt have to be smoking
> fast so a flashdrive that is readyboost capable(only a measurement of
> speed) should be fine.
>
> I think that you have to make the journal device during filesystem
> creation but google might tell you that you can add an external
> journal later, im not 100% sure on this.
It is possible to relocate the journal after-the-fact. It's not
possible to re-integrate it.
Chris
* [root@archive-1 ~]# xfs_info /data
meta-data=/dev/drbd0 isize=256 agcount=32,
agsize=25165760 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=805303520, imaxpct=15
= sunit=64 swidth=832 blks, unwritten=1
naming =version 2 bsize=4096
log =external bsize=4096 blocks=32768, version=2
= sectsz=512 sunit=64 blks, lazy-count=0
realtime =none extsz=4096 blocks=0, rtextents=0
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