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Yet another "first time RAID questions" post

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I'm about to use RAID for the first time with two SSDs in RAID 1 and two HDDs in RAID 1. I'm leaning heavily towards the LSI 9260-8i. I'm curious about the following:

1) How does a RAID controller detect issues with a drive? I'm assuming it does more than just poll the drive's SMART values periodically, right?

2) Does write caching in the controller increase performance in RAID 1 configurations by a little, a lot or not at all? I've heard it can help a lot with RAID 5 and other levels but was curious about RAID 1.

3) I've read a lot about the downsides of using consumer HDDs in arrays due to their lack of TLER. What about SSDs...do they have a TLER mechanism? Or are they generally so quick that they don't need such a mechanism, even when failing?

Thanks!

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I'm about to use RAID for the first time with two SSDs in RAID 1 and two HDDs in RAID 1. I'm leaning heavily towards the LSI 9260-8i. I'm curious about the following:

1) How does a RAID controller detect issues with a drive? I'm assuming it does more than just poll the drive's SMART values periodically, right?

2) Does write caching in the controller increase performance in RAID 1 configurations by a little, a lot or not at all? I've heard it can help a lot with RAID 5 and other levels but was curious about RAID 1.

3) I've read a lot about the downsides of using consumer HDDs in arrays due to their lack of TLER. What about SSDs...do they have a TLER mechanism? Or are they generally so quick that they don't need such a mechanism, even when failing?

Thanks!

For #1, generally RAID controllers will detect SMART errors and report back. If its a bad enough error it might drop it out of the RAID or at least give you some warning. I'll cross check that though to make sure.

Writing caching can never hurt, unless you have a power failure and you dont have a BBU or a UPS. Its acting as additional cache in front of the drive, so even a RAID0 of one drive would benefit.

We haven't seen that topic come up yet, and there are many enterprise projects using consumer-class SSDs in RAID that work fine.

Which SSDs were you planning on using? That RAID card is probably your best bet... dead reliable and able to perform at levels higher than you'd probably want to throw SSDs at.

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FWIW, the LSI controller you list is now 2 generations behind current, the 9270-8/i9271-8i is the current one-- although to be fair if this is for harddisks only, I doubt you'll see much of a performance difference between the three (or the intervening 9265-8i/9266-8i).

RAID controllers can also initiate array integrity checks, usually by the user running them manually or setting up an automated schedule.

Write caching depends on the application workload, obviously a sequential workload beyond cache size may not help much, but yes, you will see gains. If you believe you are in a cache-sensitive situation then picking up a 9265-8i/9266-8i (1 generation old) or the current cards may be additional beneficial as they have 1GB cache instead of 512MB cache as on the older card. In our internal benchmarks several years ago (LSI 9260-8i-era and older in parity RAID configurations) we saw significant gains in some areas going from 512MB cache to 2GB cache-- however these were specific to our test conditions and probably do not reflect your actual workload.

SSDs-- yes, they can have issues. That said most makers (Adaptec, LSI/3ware, and Areca) now are pretty good about compatibility testing, so read the compatibility reports.

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For #1, generally RAID controllers will detect SMART errors and report back. If its a bad enough error it might drop it out of the RAID or at least give you some warning. I'll cross check that though to make sure.

Writing caching can never hurt, unless you have a power failure and you dont have a BBU or a UPS. Its acting as additional cache in front of the drive, so even a RAID0 of one drive would benefit.

We haven't seen that topic come up yet, and there are many enterprise projects using consumer-class SSDs in RAID that work fine.

Which SSDs were you planning on using? That RAID card is probably your best bet... dead reliable and able to perform at levels higher than you'd probably want to throw SSDs at.

By SMART errors, do you mean the attributes that you can poll using programs like HD Tune? Or do you mean something more proactive / immediate, like the controller noticing when the drive returns an error when reading/write a sector?

I've also looked at SMART attributes enough to know that they aren't always indicative of a failure and sometimes require a bit of subjective interpretation. So I'm hoping RAID controllers do more than just poll the SMART attributes.

The SSDs I'm looking at are the Plextor M3 Pro - I'll be using a pair of 512GB models on one box and a pair of 256GB models on the others.

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