A quick one about cache drive!

[Draven]

Dear fellows unRAIDer,

 

I have a quick noob question.  I would like to order another 3TB hard drive, preclear it and have it ready in case of a data or parity drive failure.  Lesser my RAID will run in a degraded state, better I'll be.  But, I'm not a big fan of "sleeping" money so, I'll probably use this drive as a cache drive.  So my question is, will it be possible to "uncache" it and use it to replace a data drive in case of a failure?

 

Thank you

Draven

[mr.sparkle]

Yes.

[sureguy]

It can't be used to replace a failed data drive if the parity drive is smaller than 3tb

[mr.sparkle]

Yeah, I should add that the cache drive should be at least as big as the drive you want to replace. Also, you should preclear the drive before assigning it as a cache. You don't strictly have to do so but it'll save time later on if another one fails.

[Draven]

Thank you all for your replys,

 

Indeed, I forgot to mention that all my drives are exactly the same (5 WD30EFRX (1 parity + 4 data drives)).  The cache drive will be another WD30EFRX.  And I always preclear my drive at least one time each.  So, if I understand well, it's a YES.

 

Thank you

Draven

[garycase]

"...  Also, you should preclear the drive before assigning it as a cache. You don't strictly have to do so but it'll save time later on if another one fails. " ==>  While pre-clearing the drive will indeed do a good test on the drive to confirm it's okay before adding it to your system;  it will NOT "save time later" if you have to "un-cache" it and assign it as a data drive.    Once you've used it as a cache drive, it no longer is a zeroed drive that can be added to the protected array without first clearing it.   

 

Pre-clearing also doesn't speed up a drive rebuild ... but again it's a good test of the drive before putting it in service.

 

As for your idea of using your "spare" as a cache ==> I wouldn't.    For a couple of reasons:

 

(1)  The most-used drives in the system are the cache and parity drives.  The idea of having a spare is that it's a good, new drive ready to be used at a moment's notice.    When a drive fails, you want a NEW drive for the rebuild -- not one of the most heavily used drives in the system that have the same number of hours on them as the failed drive.

 

(2)  3TB is serious "overkill" for the cache drive -- you don't need anywhere near that amount of space (just how much do you plan to write to the array in a day ?)      Personally, I don't use a cache drive (I want all of my writes to the protected array to be protected as soon as they're written to the array -- not a few hours later when the mover runs).    But if I did add a cache, it'd be a relatively small unit that I have "laying around" -- perhaps 250GB;  or even a small SSD.

 

I'd pre-clear your extra 3TB drive to confirm it's good; then just store it until you need it.

 

[dgaschk]

The thread pinned to the top of this forum that discusses the merits of using a cache drive as a spare.