[mythtv-users] OT: Hybrid drives in a RAID?

[Andre Newman]

On 20 Aug 2014, at 14:47, Matt Emmott <memmott at gmail.com> wrote:

> There's a deal on slickdeals.net this morning where one can buy a 4TB Seagate "Hybrid drive" (model ST4000DX001) for somewhere in the neighborhood of $150 shipped. This drive has 8GB of NAND flash built in, and from what I've read performs block-level offload of the frequently accessed data to the flash, giving it (in theory) a performance boost similar to SSD.
> 
> I'm planning on building a 6-disk RAID6 array in my shiny used Poweredge once it shows up. If hybrid disks offer a speed boost at an affordable price, it seems like a no-brainer to use them for my array. However, I can't find any solid information on using them in a RAID5 or RAID 6 setup. I did find this quote from HP via http://h10025.www1.hp.com/ewfrf/wc/document?docname=c03821563&cc=us&dlc=en&lc=en :
> 
> >An SSHD in RAID 0, as compared to a single drive configuration, delivers performances as >much as 70% higher.
> >Because current drives are not designed for multi-drive vibrations, they have not been tested >with RAID 5, 6, or 10, and are not recommended.
> 
> I'm going to ignore the last part about multi-drive vibrations because I assume it's some CYA warranty fluff, and danger is my middle name. But, I don't know what I don't know so I'm asking the smart people on this list if they think a hybrid drive would be a benefit, or a deterrent, or something else when running it in a RAID5 or 6 setup.

I wouldn’t bother with the Seagate Hybrid drive (too complicated to use) but I do run several systems with hybrid SSD HDD arrays, including the MythTV server at home.

There are various ZFS and BTRFS configurations that can benefit from SSDs in the array but I don’t have much experience with those, yet...

I’ve used mdadm Raid 1 with “write mostly” flags set on the HDD elements, either a three way raid1 with an SSD sized “fast” array and a regular “slow” HDD only array using the remaining disk space or an SSD with a partially used HDD used to raid the faster device.

The reference I used for this technique is here:
http://tansi.info/hybrid/

Adaptec do something similar:
http://www.adaptec.com/en-us/solutions/hybrid-raid/

I’ve had a couple of these systems in heavy use for about 18 months and no problems to report so far. Performance is similar to SSD for short bursty use, as a regular desktop it seems very quick, for Myth the database always seems quick to respond.

I have a mail server that uses it (60GB SSD +2x 1.5TB HDD) and mail folder refreshes and sorts are very rapid, that server doesn’t do much else that would be noticeably faster.

For heavier uses like a big compile it slows down to the HDD speed quite quickly.

I think it’s a useful technique in it’s place.

Andre




> 
> Thoughts? Thanks in advance.
> 
> Thanks,
> Matt
> 
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