[opensuse] Re: RAID-1 speed -- RAID 1 for? Backup?
Linda Walsh skrev:
Anders Norrbring wrote:
I'm wondering about the speed of a software RAID-1... If I use one very fast disk, and one pretty much slower, will the volume's speed be the faster or the slower? If using an ordinary hardware RAID most controllers gives the volume the highest speed possible, and then mirror in the background to the slower disk, but is the same true for a software RAID?
Anders.
Most likely NO FASTER than the slowest -- it might be slower due to them being out of sync with each other on writes.
You really don't want to use mismatched drives for a RAID. Use your faster one as a primary and use the 2nd one as a backup disk -- if you use xfs on drive 1, you can use xfs_dump to dump all extended file attributes and access lists, as well as telling it what files or directories not to include with the +d file_attr.
You can use a tower-of-hanoi backup sequence to optimize space and restore time -- and, of course, optionally compress the files when storing them into the backup. Run it as a nightly cron job...
Of course you probably aren't wanting this because you already have a daily backup system in place, right? I mean everyone know that comes before considering a mirrored RAID.
:-) -linda
Hi Linda.. Yes, I do nightly backups of the host system... The reason for considering the mirror is that it's virtual machines running on VMware and the host system is using a very high speed SAS array and I simply would like a hot backup "just in case". It's really not a big deal since the SAS array is RAID-5 with 2 hot spare drives, so I'm pretty certain drive failures won't be a problem, however the risk of disk trashing is always present in case of a "unclean" shutdown. Then a hot mirror of the VM drives can come in handy, at least for the IMAP and MySQL functions. I think I'll simply do some experimenting on the matter.. ;-) Thanks for your feedback! Anders. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org
Anders Norrbring wrote:
It's really not a big deal since the SAS array is RAID-5 with 2 hot spare drives, so I'm pretty certain drive failures won't be a problem,
You meant to say "I'm certain drive failures will be a problem" ? :-) A key issue with a RAID1 or -5 is the time it takes to recover. Whilst the array is recovering, you're very much exposed. A RAID5 built of 1Tb drives could take quite a while to recover from a one drive failure. One way of reducing the risk is to use drives with staged manufacturing dates, i.e. not all from the same batch/date. /Per -- Per Jessen, Zürich (8.1°C) -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org
Per Jessen skrev:
Anders Norrbring wrote:
It's really not a big deal since the SAS array is RAID-5 with 2 hot spare drives, so I'm pretty certain drive failures won't be a problem,
You meant to say "I'm certain drive failures will be a problem" ? :-)
A key issue with a RAID1 or -5 is the time it takes to recover. Whilst the array is recovering, you're very much exposed. A RAID5 built of 1Tb drives could take quite a while to recover from a one drive failure. One way of reducing the risk is to use drives with staged manufacturing dates, i.e. not all from the same batch/date.
/Per
Haha.. ;-) Well, we all know drive failures WILL happen at some time.. But with a hardware SAS RAID-5 and 2 hot spares, I don't really see that it would create any acute problems besides possibly the rebuild time which is around 2 hours for a failed drive in my case. And yes, all the 8 drives are from different batches and dates. That's not really the issue here, I was more thinking of a "hot mirror backup" of the virtual machines, but as I mentioned, I'll do some experimenting with the speeds. I'll post back my findings when done. Anders. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org
Haha.. ;-) Well, we all know drive failures WILL happen at some time.. But with a hardware SAS RAID-5 and 2 hot spares, I don't really see that it would create any acute problems besides possibly the rebuild time which is around 2 hours for a failed drive in my case. And yes, all the 8 drives are from different batches and dates.
"Battle Agaist Any Raid F*" <http://www.baarf.com/> Interesting reading by people who know a *lot* about storage. -- OpenGroupware developer: awilliam@whitemice.org <http://whitemiceconsulting.blogspot.com/> OpenGroupare & Cyrus IMAPd documenation @ <http://docs.opengroupware.org/Members/whitemice/wmogag/file_view> -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org
Anders Norrbring skrev:
Per Jessen skrev:
Anders Norrbring wrote:
It's really not a big deal since the SAS array is RAID-5 with 2 hot spare drives, so I'm pretty certain drive failures won't be a problem,
You meant to say "I'm certain drive failures will be a problem" ? :-)
A key issue with a RAID1 or -5 is the time it takes to recover. Whilst the array is recovering, you're very much exposed. A RAID5 built of 1Tb drives could take quite a while to recover from a one drive failure. One way of reducing the risk is to use drives with staged manufacturing dates, i.e. not all from the same batch/date.
/Per
Haha.. ;-) Well, we all know drive failures WILL happen at some time.. But with a hardware SAS RAID-5 and 2 hot spares, I don't really see that it would create any acute problems besides possibly the rebuild time which is around 2 hours for a failed drive in my case. And yes, all the 8 drives are from different batches and dates.
That's not really the issue here, I was more thinking of a "hot mirror backup" of the virtual machines, but as I mentioned, I'll do some experimenting with the speeds. I'll post back my findings when done.
Anders.
I've been running bonnie++ 5 times on each individual drive and on the mirrored volume, and from what I can see, the mirror set is slightly slower than the fastest drive (about 85% of the single drive speed) but not close to as slow as the slower drive. My conclusion is that the software RAID in fact do mirror/buffer in the background to keep the speed up. Anders. -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org
participants (3)
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Adam Tauno Williams -
Anders Norrbring -
Per Jessen