HDD Cache - 256 MB vs. 512 MB?

TrueNAS General
1

Hello,

how important is a harddrives internal Cache size for TrueNAS / ZFS?
Is a harddrive with 512 MB Cache much faster than a drive with 256 MB Cache?
Is there a huge drawback if drives with 256 MB Cache and 512 MB Cache are mixed on the same RAIDZ3?
For what exactly will a harddrive use the Cache? Is it for reading larger blocks from the platters and putting them into the cache or will it also be used as “write cache”?

My system will be used as a homeserver used for storage/archive, usually without any high workload.

Thanks,

Thomas

2

Careful with drives that have a higher internal cache than their cousins of the same capacity, as they could be SMR drives.

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3

I am aware of SMR drives, but thanks for the hint.

Any further information on the HDD cache sizes?

4

Unless someone can explain otherwise, you probably won’t notice much of a difference between the two drives when using ZFS.

The ARC (which is the ultimate “read cache” in RAM) is massive in comparison to the HDD’s internal cache, and bypasses SATA/SAS altogether. (Directly read from RAM.) The ARC can be measured in the tens (or hundreds) of gigabytes.

You might theoretically get slightly better response time from small random reads with an HDD that has a larger internal cache, but possibly won’t notice much of a difference in the real world between 256 MiB vs 512 MiB when using ZFS.

And to reiterate, when comparing two “similar” harddrives (less than 8 TiB capacity), be wary of the one with a much larger internal cache. (A larger cache is needed by SMR drives, due to their design.)

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