Re: [offtopic] write cache size on hardware raid controllers

On 28/01/2021 10.13, Per Jessen wrote:

On 28/01/2021 10:02, Andrei Borzenkov wrote:

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On Thu, Jan 28, 2021 at 10:40 AM Per Jessen wrote:

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The built-in cache on the disk drive is for speeding up reads, not writes.

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And begs the question about who backs up the hard disk buffer memory.

It is not used for write caching.

Are you sure? I understood that the on disk cache, by default, is read/write.

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It is. Unless HDD has internal non-volatile backup (some have) it should really be disabled, and this is normally default when used with RAID controllers - except some vendors lie and retain write cache. Good for benchmarks ...

Ah.

Yeah, you're right, I know there is a setting to enable it.  We check every new drive when they are installed, just in case - we have not seen any IBM/Hitachi/Seagate/WDC drive with write cache enabled in at least 10 years.  I don't know if we ever have, but we don't keep the test records for longer than that.

Ok, you mean that hard disks that are specifically made for raid usage, have the write cache disabled? Or that you manually disable it? I don't know, I never purchased disks made for raid. And then, the same disks for normal usage have the write cache enabled? If I do "hdparm -I" on a normal disk of this computer, I get: Configuration: ... cache/buffer size = unknown ... Commands/features: Enabled Supported: * SMART feature set Security Mode feature set * Power Management feature set * Write cache <=== -- Cheers / Saludos, Carlos E. R. (from 15.2 x86_64 at Telcontar)