The biggest thing to be careful of with this kind of deployment is that
now a single drive failure will take out 2 OSDs instead of 1 which means
OSD failure rates and associated recovery traffic go up. I'm not sure
that's worth the trade-off...
Mark
On 07/08/2015 11:01 AM, Quentin Hartman wrote:
Regarding using spinning disks for journals, before I was able to put
SSDs in my deployment I came up wit ha somewhat novel journal setup that
gave my cluster way more life than having all the journals on a single
disk, or having the journal on the disk with the OSD. I called it
"interleaved journals". Essentially offset the journal location by one
disk, so in a 4 disk system:
OS disk sda has journal for sdb OSD
sdb OSD disk has journal for sdc OSD
sdc OSD disk has journal for sdd OSD
sdd OSD disk has no journal on it
This limited the contention substantially. When the cluster got busy
enough that multiple OSDs on the same machine were writing
simultaneously it still took a hit, but it was a big upgrade from the
out of the box deployment. I also tried leaving the OS drive out and
only interleaving the journals on the OSD drives, but that was slightly
worse under load than this configuration. It seems that the contention
of the journals and OSDs was stronger than the contention with logging.
QH
On Fri, Jul 3, 2015 at 1:23 AM, Van Leeuwen, Robert
<rovanleeuwen@xxxxxxxx <mailto:rovanleeuwen@xxxxxxxx>> wrote:
> Another issue is performance : you'll get 4x more IOPS with 4 x 2TB drives than with one single 8TB.
> So if you have a performance target your money might be better spent on smaller drives
Regardless of the discussion if it is smart to have very large
spinners:
Be aware that some of the bigger drives use SMR technology.
Quoting wikipedia on SMR:
"shingled recording writes new tracks that overlap part of the
previously written magnetic track, leaving the previous track
thinner and allowing for higher track density.”
and
"The overlapping-tracks architecture may slow down the writing
process since writing to one track overwrites adjacent tracks, and
requires them to be rewritten as well."
Usually these these disks are marketed "for archival use".
Generally speaking you really should not use these unless you
exactly know which write workload is hitting the disk and it is just
very big sequential writes.
Cheers,
Robert van Leeuwen
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