Re: Disk I/O slowness

On Thu, Jul 30, 2026 at 07:43:36PM -0700, David Christensen wrote:
>Perhaps the MegaRAID card caches writes to battery-backed RAM, reports 
>synchronous write completion to the OS, and then writes the cache to 
>the disk drives later (?).  This would allow OS and application 
>synchronous writes to complete quickly.

Yes, raid 5/6 is basically only usable for write-heavy loads if there's 
a write cache. A single block write requires a read of any other blocks 
in a stripe in order to generate checksum data. In an application that 
syncs data to disk that turns a series of writes into 
read-write-read-write-read-write which will kill performance. With a 
non-volatile cache that remains write-write-write (assuming the writes 
can be consolidated so an entire block + parity can be written without 
needing to read in stale data) or at least hides the latency issues if 
the cache doesn't fill completely. You could theoretically cache the 
writes without the nv memory, but the chances of corruption on power 
failure or crash are very, very high.

As others have suggested, a small array on consumer hardware is much 
more useful as a raid 1 than a raid 5.