RaidZ2 HDD varying cache sizes

[Bill McCormick]

I've got a mish-mash of HDD's (WD REDs, PRO's, BLACK's) in a raidz2 config. They're all the same size and speed, but the disk cache varies from 64MB to 256MB. When I built this system I either didn't care or didn't know enough to care about it. So now I'm wondering: is this cache size spread an issue in any way?

Thanks!

 

[sretalla]

is this cache size spread an issue in any way?

Probably a little, but of no real consequence.

The pool will be held back by the drives with the smaller cache, so if the drives with larger cache could be performing better, they won't in the current setup, but that's it.

If you find that things run well, nothing to be concerned about.

 

[Etorix]

So now I'm wondering: is this cache size spread an issue in any way?

It could be a major concern if some of the WD drives with 256 MB cache are actually SMR drives. In that case they should NOT be used in RAIDZ2 with ZFS! Please check your models:

List of known SMR drives

Hard drives that write data in overlapping, "shingled" tracks, have greater areal density than ones that do not. For cost and capacity reasons, manufacturers are increasingly moving to SMR, Shingled Magnetic Recording. SMR is a form of PMR...

www.truenas.com

 

[sretalla]

It could be a major concern if some of the WD drives with 256 MB cache are actually SMR drives

Good point... some of the REDs could be EFAX, so would be SMR. I don't see Black drives on the list (although nobody would expect to see those, so maybe they avoided the list for other reasons than not being SMR) and there are no PRO WD drives that are SMR.

 

[Bill McCormick]

Yes. Excellent point. Looks like I'm okay though:

Code:

/dev/da0,Model Family: Western Digital Red Pro,Device Model: WDC WD6002FFWX-68TZ4N0,Cache: 128 MB
/dev/da1,Model Family: Western Digital Black,Device Model: WDC WD6003FZBX-00K5WB0,Cache: 256 MB
/dev/da2,Model Family: Western Digital Red Pro,Device Model: WDC WD6002FFWX-68TZ4N0,Cache: 128 MB
/dev/da3,Model Family: Western Digital Red Pro,Device Model: WDC WD6003FFBX-68MU3N0,Cache: 256 MB
/dev/da4,Model Family: Western Digital Red,Device Model: WDC WD60EFRX-68MYMN1,Cache: 64 MB
/dev/da5,Model Family: Western Digital Red Pro,Device Model: WDC WD6003FFBX-68MU3N0,Cache: 256MB
/dev/da6,Model Family: Western Digital Black,Device Model: WDC WD6003FZBX-00K5WB0,Cache: 256 MB
/dev/da7,Model Family: Western Digital Red Pro,Device Model: WDC WD6003FFBX-68MU3N0,Cache: 256 MB

 

[jenksdrummer]

You have a mix of 5400 and 7200RPM drives as well, as well as Black(or Blue or Green) drives which don't handle TLER. Generally that matters with a RAID Controller; but not sure how FreeNAS handles that kind of thing via SMART.

Your best approach would be to keep like for like together in the same array; otherwise you start circling the drain as to what goes away from known-working into good luck!