USB3.0flash drive as Z2ARC

[andoy31]

I'm contemplating on whether i'll use USB 3.0 flash disk as Z2ARC or just buy an SSD... Basically i'm looking at using 32GB USB3.0 as Z2ARC, using USB3.0 will save me a SATA port that i can use for further ZFS storage expansion

any thoughts?

 

[William Grzybowski]

USB is never a good idea coming to storage in my honest opinion... I'd just buy a sata SSD.

OBS: It is called L2ARC, not Z2ARC.

 

[jgreco]

USB is never a good idea unless it's completely disposable without penalty, and guess what, for L2ARC, it basically is.

Lots of people use USB flash as a cheap substitute for a real SSD, though the popularity in doing so seems to be dropping now that SSD's are affordable by mere mortals.

Considerations:

1) The flash may be considerably slower than an SSD, even if it is for "USB 3.0", because your typical SSD is made up of an array of flash chips that are then effectively RAID0'd by the SSD controller, giving lots of parallel access to many slower chips. Your USB flash drive may only have a single flash chip.

2) The performance should be tested. There's no point in a USB L2ARC if it isn't giving you some performance gains (especially since it could be hurting you).

 

[andoy31]

Was thinking of two things at once hence the z2arc (zfs and l2arc), sorry about that.

Hmmm, if an l2arc were to have some disk reliability type of problems- what impact does it have on the freenas/zfs?

 

[jgreco]

As a practical matter? You might need to detach it if things go wrong. If the OS gets hung up by a misbehaving device, that could be a bit of a problem if it means your production fileserver starts acting wonky. I'm guessing that FreeBSD is sufficiently robust these days, and the device drivers are sufficiently mature, that it wouldn't be a real problem. I don't think I'd have considered USB storage years ago, but now it is pretty common.