Parity speed

[Monnier Adrien]

Good morning,

 

I have a array of 8 4TB disks and the parity check runs at 107MB/s, is this a normal speed? Because I find it a little slow.

 

I do not have containers or VMs on the array and I ran a benchmark with diskspeed on each disk and there is no problem with speed, they run at around 200MB/s.

 

I have 5 disks on the SATA controller of the motherboard and 3 disks on a SATA controller in PCI-E.

[MAM59]

Parity runs maximum of the speed of the slowest drive (up to the block where this drive ends, then the next slowest will be the limit), it even slows down a lot (~30-50%) at the end, just because the inner sectors come in more slowly.

So if you see 200MB/s of a drive on the outside, it is likely to have only 150 or less on the inside.

Also, the more disks you have, the longer it takes to read them (naturally) and to calc & check the parity.

UNRAID demands that the parity drive must be the biggest one (so that every block on a data drive has a corresponding block on the parity drive), they forget to tell you that it also should be the fastest one if possible. Because parity is envolved in every write operation, it will slow down every (write) command too.

 

But back to your check.... yeah 50% of the max speed of the drives is usually a "good" value, your 107MB/s do look ok.

(Here Disks can run up to 280Mb/s and the check gives 201Mb/s each time. But if I add an old 10Tb Drive to the array, it instantly falls down to 100MB/s check time (no more 10Tb drives in there now 😁))

[Monnier Adrien]

Thank you for this clear and reasoned answer, I understand the speed of 107MB/s better

[sota]

 

starts at 218, drops to 103 at the end.

my last parity check of 5+1 drives...

Duration: 18 hours, 9 minutes, 19 seconds. Average speed: 153.0 MB/sec

Edited December 13, 2023 by sota