[gpfsug-discuss] Client Latency and High NSD Server Load Average

[Valdis Kl=?utf-8?Q?=c4=93?=tnieks]

On Thu, 04 Jun 2020 15:33:18 -0000, "Saula, Oluwasijibomi" said:

> However, I still can't understand why write IO operations are 5x more latent
> than ready operations to the same class of disks.

Two things that may be biting you:

First, on a RAID 5 or 6 LUN, most of the time you only need to do 2 physical
reads (data and parity block). To do a write, you have to read the old parity
block, compute the new value, and write the data block and new parity block.
This is often called the "RAID write penalty".

Second, if a read size is smaller than the physical block size, the storage array can read
a block, and return only the fragment needed.  But on a write, it has to read
the whole block, splice in the new data, and write back the block - a RMW (read
modify write) cycle.
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