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Software RAID5
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Author:  Spreadie [ Mon Sep 21, 2015 12:50 pm ]
Post subject:  Software RAID5
I have three 1TB HDDs in RAID5 on my Gen7 and I have a DAS box with a 2TB HDD connected via eSATA for periodic backup.

Now, before I fill the thing with data, I was struck by how long it took to initialise the array (more than five hours), and I'm thinking about buying a P410 RAID card to take some load off the CPU. I can probably grab a P410 for £30-£40 on feebay, and they are proven to work with the Proliant Microservers, but is it worth the effort and outlay?

Can I expect to see reduced rebuild times and improved write speeds on a low end hardware RAID5 array compared to a software array? I'm thinking of swapping out the HDDs for 2TB models in the future, and I'd rather not have to leave the system rebuilding for half a day.

Opinions?
Author:  jonbwfc [ Mon Sep 21, 2015 1:48 pm ]
Post subject:  Re: Software RAID5
Spreadie wrote:
I have three 1TB HDDs in RAID5 on my Gen7 and I have a DAS box with a 2TB HDD connected via eSATA for periodic backup.

Now, before I fill the thing with data, I was struck by how long it took to initialise the array (more than five hours), and I'm thinking about buying a P410 RAID card to take some load off the CPU. I can probably grab a P410 for £30-£40 on feebay, and they are proven to work with the Proliant Microservers, but is it worth the effort and outlay?

Can I expect to see reduced rebuild times and improved write speeds on a low end hardware RAID5 array compared to a software array? I'm thinking of swapping out the HDDs for 2TB models in the future, and I'd rather not have to leave the system rebuilding for half a day.

Opinions?

With write speeds the cache is the main differentiator. Obviously a software RAID will use your PC's memory as cache, where as the hardware card will have it's own RAM. However data will have to go into the RAID CARD cache via the PCI bus, which will almost inevitably be slower than the bus to main system RAM. It may well depend on exactly what memory you have in t he PC (frankly, a 30 quid RAID cardisn't like to have very fast cache memory on it). I suspect rebuild times will be pretty much similar, as in that case the bottleneck will end up being the limit of the actual drives themselves.
Author:  saspro [ Mon Sep 21, 2015 4:47 pm ]
Post subject:  Re: Software RAID5
It's worth getting a card to take the load off the CPU.
But rebuilds do take time. Even on a SAN with GB's of RAM & dedicated controllers it can take days or weeks to rebuild an array.
My 30TB backup box took about a week to get in sync
Author:  Spreadie [ Mon Sep 21, 2015 5:02 pm ]
Post subject:  Re: Software RAID5
saspro wrote:
It's worth getting a card to take the load off the CPU.
But rebuilds do take time. Even on a SAN with GB's of RAM & dedicated controllers it can take days or weeks to rebuild an array.
My 30TB backup box took about a week to get in sync

I was expecting lengthy build times, but thought 5+ hours seemed excessive just to initialise a blank array that consisted of three 1TB drives.
Author:  saspro [ Mon Sep 21, 2015 6:40 pm ]
Post subject:  Re: Software RAID5
Spreadie wrote:
saspro wrote:
It's worth getting a card to take the load off the CPU.
But rebuilds do take time. Even on a SAN with GB's of RAM & dedicated controllers it can take days or weeks to rebuild an array.
My 30TB backup box took about a week to get in sync

I was expecting lengthy build times, but thought 5+ hours seemed excessive just to initialise a blank array that consisted of three 1TB drives.


It's about standard unfortunately. It's a bit like accidentally doing a full format on a big hdd rather than a quick format.

It's why most people on RAID controllers do a quick init then let it finish later
Author:  Spreadie [ Tue Sep 22, 2015 8:41 am ]
Post subject:  Re: Software RAID5
saspro wrote:
Spreadie wrote:
saspro wrote:
It's worth getting a card to take the load off the CPU.
But rebuilds do take time. Even on a SAN with GB's of RAM & dedicated controllers it can take days or weeks to rebuild an array.
My 30TB backup box took about a week to get in sync

I was expecting lengthy build times, but thought 5+ hours seemed excessive just to initialise a blank array that consisted of three 1TB drives.


It's about standard unfortunately. It's a bit like accidentally doing a full format on a big hdd rather than a quick format.

It's why most people on RAID controllers do a quick init then let it finish later

I don't think I had a quick initialise option in the software. For the sake of sparing the CPU I may just grab a card anyway - I will be serving media off this box too, so it won't hurt. I have 8GB of RAM coming too.

Thanks
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