What kind of performance benefit write back on a RAID controller gives?

To be more specific, the setup is 2 SSDs in RAID 1. The workload is mainly random write.

Given the above, what kind of performance increase can I expect using write back vs write through on a RAID card with 1GB RAM?

More information:

OS: Cent OS 6.5
CPU: Intel Xeon E3-1240v2
SSD: Intel S3500 160GB
RAID controller: MegaRAID SAS 9271-8i

  • 1
    Brand of server, make/model of RAID controller, brand/model of SSD... Some basic information that will make this a better question and help us provide good answers.
    – ewwhite
    Mar 24 '14 at 22:56
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    I assume you know that write operations are no faster on RAID-1 than to a single disk (can even be slower for rotating media) - and that your not getting better reliability than a single disk - the expected failure mode for SSDs is write-wear that both the disks will expire at the same time unless you buy three and swap one out half way through its duty cycle.
    – symcbean
    Mar 24 '14 at 23:29
  • I know write operations are a bit slower depending on the controller, but I believed by buying 2 SSDs and RAID 1 them I get better reliability. I see your point regarding write-wear, but I believe this covers me in case of unexpected failures. This is provided by a hosting company, so I have no way to know how much data were written on an SSD.
    – Luka
    Mar 24 '14 at 23:33

For that specific hardware combination, the controller cache may not provide much help.

  • The Intel SSDs you're using provide power-loss protection, so the BBWC's function may be redundant. I doubt you'd see a difference with and without cache with those SSDs.
  • The write IOPs capacity of those SSDs should absorb a random write workload well.

The best performance enhancement you can make here is to leverage your LSI controller's FastPath functionality. This optimizes the data path for SSDs instead of the algorithms tuned for rotational media.

  • When you say not much performance difference, I understand it's very hard to estimate performance, but can you be a bit more specific, eg up to 20% or so, if possible please? Also, this is provided by a hosting company and the don't provide FastPath.
    – Luka
    Mar 24 '14 at 23:24
  • On controllers of that generation, I don't enable battery or flash-backed caching on volumes comprised of SSDs. So I can't tell you how much your application will benefit or not from that configuration. I can tell you that it's moot because FastPath is on a different order of performance increase.
    – ewwhite
    Mar 24 '14 at 23:43
  • Thanks for pointing this out. Above you write "BBWC's function may be redundant.". If I use write back and don't use BBU, Will I be safe, given the SSDs provide power loss failure?
    – Luka
    Mar 24 '14 at 23:46

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