Some questions about VRAID

[Oscar_Maimó]

Hi guys:
I have some questions to do:
What are the differences between RAID 10 and RAID 0+1?

Which of they are the equivalent to VRAID1 in EVA?

When a device writes data to Disks in EVA, it do it in blocks of 128 KB(chunk) or in blocks of 2MB (PSEG) per disk?

Greetings
Oscar

[S. Boetticher]

RAID10: mirror disks in pair (R1), then stripe across all those mirrors (R0)

RAID0+1: create two Stripesets (R0) across lot's of disks and then mirror (R1) those two stripesets.

R10 is IMHO better, because you can loose any disk up to half of the disks, as long as never 2 mirrored disks of a pair fails at the same time.

R0+1: if the first disk dies, then the whole stripe is gone, so the first disk from the other stripe dying will kill all.

so for R10 you have a higher probability of survival of several disks, however for both the guarantee is only 1 disk!

EVA VRaid1 thus is more a R10, because the data is mirrored first, then striped

chunk or PSEG: I'm not really sure, but I believe it was PSEG...

[Oscar_Maimó]

Thanks for response but i would like more information about chunks an PSEG.
Greetings
Oscar

[Uwe Zessin]

Only a developer who has access to the engineering data can definitely tell, but I am very sure the EVA does write as much data as it receives from the host: if the host sends a single 512 byte block, the EVA writes exactly this block to the desired location on the disk drive. The physical disk drives allow this granularity, so no blocking/de-blocking is necessary.

If you are looking for the maximum physical I/O size - that depends how 'clever' the firmware/hardware is: whether it can scatter/gather a single host write that is larger than 4 chunks and write it into a PSEG. If not, it will split it up into 128KB physical writes, because the EVA always puts 4 adjacent logical chunks onto different disk drives.

Again, only somebody with deep, deep insight into the architecture can tell for sure.

[Oscar_Maimó]

Suppose that a host writes a 256 kB block.

Does the block is divided in two 128 kB blocks in to different disks? or are situated in the same disk drive?

Greetings
Oscar

[Richard Tengdin · Accepted solution]

The EVA uses a 128KB chunk size and a 512KB Stripe size when writing to disk. This is below the 8 MB Rseg level.

128KB of data is written to a mirror pair in the RSS and 4 pairs are striped together to get the 512KB stripe. The full 8 MB in an Rseg consists of 16 data stripes. The next 8MB of data will be written to the next RSS/Rseg in the Disk Group. Essentially the EVA is doing concatenated striped mirrors.

In your example (assuming block-aligned writes) a 128KB write will go to two disks and a 256KB write will hit 4.

[Sivakumar MJ._1]

Hi Oscar,

Check this PPT for your understanding..

[Oscar_Maimó]

Thanks guys:
I learned some things that i didn't know
Greeting
Oscar