Re: Mirrored volume peformance questions

[Roberto Spadim <roberto@spadim.com.br>]

2011/5/3 Morad, Steve <morad@amazon.com>:
> I have a few questions about volume mirroring performance implications.
>
> 1. I'm looking for an optimal configuration to maximize read speed while protecting the data from a disk failure (e.g. for an application that primarily does reads).  For RAID 1, does the linux raid driver perform parallel read requests across all available disks?  If, for example, I set up a RAID 1 volume with 4 disks, would the OS handle concurrently reading (different) data from all 4 disks at the same time?

today read_balance algorithm is optimized for multi-thread
read algorithm preffer disk with minimal head distance (current block
- block that will be read)

you can unselect disks setting they as write-mostly

optimizations at raid1 aren´t for bigger sequencial read, they are for
multi-thread



> 2. Similarly, would a RAID10 configuration give me the same (or better) read behavior across these same disks, while providing twice the storage capacity of the above configuration?

in md world
raid1+ raid0 != raid10

raid10 can use layouts
raid1 can´t

raid10 have diferent read_balance algorithms than raid1
raid10 with far layout is better optimized for sequencial read (it´s
like raid0 stripe)
raid10 with near/offset layoute are better optimized for multthread


> 3. Is the raid driver smart enough to keep track of underlying volume performance history or queue lengths and make read requests appropriately? For example, if for some crazy reason a 10K disk was mirrored with a 7K disk, would the OS make more read requests to the 10K disk than to the 7K disk, or would read requests be equally distributed across both disks?

no
i made a new raid1.c algorithm for kernel 2.6.37, that you can change
this options, you will have 1% of speed improvement in a very high
workload of read (ok it´s not a lot but i tested three times (1 hour
non sequencial test) and 1% are seeing, it´s not a error of
benchmark), you can mix SSD and HD too

code is here:
http://www.spadim.com.br/raid1/


it´s not a md-team solution
it´s a patch, more information inside the file
to compile you must compile like anyother kernel module
make drivers/md/raid1 ....

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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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