Jesper Krogh wrote:
>> I'm running spooling on a 4 drive software raid0 quite happily on a 4Gb
>> 3GHz P4D machine. The limiting factors are disk head seek time(*) when
>> running concurrent backups to 2 LTO2 drives and available SATA ports.
>> Because of that I'm considering dropping in solid state disks.
>>     
>
> I still have got to see a reasonable priced SSD' disk that can deliver 
> around 100MB/s both ways at the same time.
>   
There aren't any mechanical disks which can do it either.

Which is why I'm not trying to do that - replacing 4 RAID0 mechanical 
disks with 4 SSDs will provide similar sustained throughput to the 
mechanical RAID0, but provide _much_ better performance for anything 
where the mechanical disks had head seeking involved - such as multiple 
simultaneous input/output streams to LTO drives.

> http://www.slashgear.com/samsung-64gb-ssd-performance-benchmarks-278717/
>
>   

Make sure you compare apples with something remotely looking like 
apples. The ONLY SSds which are suitable fo this kind of use are SLCs, 
not MLCs

> I have beefed up my director with sufficient amount of memory and 
> mounted it as a "ramdisk" for spooling. That doesn't impose any 
> limitations on the 2 LTO3 drives attached.
>   
How much do you regard as "sufficient"?

100-200Gb ram and systems capable of addressing that amount of memory 
are still far more expensive than a stack of flash drives, else I'd use 
them.

My concern isn't just backup run time.

Restore times are also important and having a tape read back 1Gb, then 
seek, then pull back another 1Gb (or even 10Gb) is a significant 
penalty  over reading larger blocks when worst-case 75Tb+ restores are 
considered (25-60 days on 2 drive LTO2, dpeending on the directory 
structures being restored.)

> And spooling doesnt need any form for persistence, so its fine that its 
> gone after reboot
Indeed.

If it was practical I'd use ramdisks. Right now it's not. Apart from the 
cost factor there is very little hardware which can address more than 
128Gb of Dram. There are RAM arrays which are setup to operate as F/O 
scsi devices, but these are currently "silly money" as they're marketed 
at the world of high end, high cost databases.

In 12 months time that may change, Ram is always falling in price - but 
Flash drive pricing is falling faster,  performance/durability is rising 
at the same time and there isn't the same issue with massive address 
ranges as it just looks like more disk, vs having to change out entire 
servers at $20k a time if RAM limits are reached.

I'm not just looking at the issue of my current setup. Projects are 
already pencilled onsite which will increase storage demands by a factor 
of 20 from the current size within 12 months and I have to try and be 
ready to back that data up.





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