BBU HW RAID only for ZIL/SLOG?

[noescape]

Hello,

as you all know, SSDs as problematic in case of power failure. In some tests, 95% of enterprise SSDs lost their data. DDRdrive is expansive and not compatible with FreeBSD.

I have my HBA in IT mode, so no HW raid for my regular HDDs and want to leave it like that. But I was wondering if buying an BBU HW RAID with 512 MB RAM to be attached to only one drive could be a good ZIL/SLOG. ???

The HP controllers can be set the ratio of write/read cache. So I will set 100% to write cache.

Have somebody tried it before? What was the result? IOs ?

I am thinking about using an classic spinning sata/sas drive behind BBU, since I see no point in using an SSD behind BBU.

Thank you

 

[cyberjock]

There's rumors of it being possible. Although it's really hard to say how well it will work, how much of a performance boost it will provide, etc.

If you are really interested in a RAM based drive, the Acard ANS-9010 and ANS-9010B both are RAM drives that use SATA, so should "just work" with FreeBSD. I have a few and I've never had a problem with them. They have their own Battery and CF card for sustained power loss.

 

[survive]

Hi noescape,

Jgreco, one of the more prolific forum posters, had a thread where he discussed doing exactly this. I think he used a HW RAID controller connected to a pair of laptop drives in a mirror. As I recall he was getting really good results with this setup.

I can't seem to find the post right now, but search the forums (or maybe use google with site:forums.freenas.org) for something like "hardware raid zil jgreco" and see what you get. While you are at it read some of his posts...he has some interesting insights into how zfs works.

-Will

 

[paleoN]

I can't seem to find the post right now, but search the forums

You mean this one?

 

[jgreco]

If you are really interested in a RAM based drive, the Acard ANS-9010 and ANS-9010B both are RAM drives that use SATA, so should "just work" with FreeBSD. I have a few and I've never had a problem with them. They have their own Battery and CF card for sustained power loss.

The main problem with this is that it is on the far side of a SATA bus ... latency. The cache on a controller is potentially much faster, though that's really dependent on firmware and drivers being "quick." Picking a good controller is probably key to this. One with a great driver, write coalescing, and a large write cache would seem ideal.

On the flip side, using a RAID controller and a drive (or drives) will limit you to the throughput of the drive(s) once the cache has filled. This will almost certainly be much slower than the higher rate of speed that a RAM based drive will be able to support continuously.

I would expect the RAID controller and a HDD to be likely to handle bursts of transactions with less latency than the RAM based drive. Sustained performance prefers the RAM based drive, I'd expect. However, finding a 5.25" bay for a RAM based drive in a large storage server is a bit of a trick.

The main reason I was messing around with it was because I liked the idea of a free SLOG device. We've got ESXi and NFS here, so being able to create a useful SLOG device for no additional capex is compelling.