On 2015-05-20 12:16 PM, Dimitri Maziuk wrote:
> On 05/20/2015 01:09 PM, Bryn Hughes wrote:
>
>> What do you call it when multiple backup jobs are writing to disk
>> storage while at the same time a copy job is reading from that same disk
>> subsystem and writing to tape?
> I call the comparison FUD because compare that to multiple backup jobs
> writing to tape at the same time while a copy job is reading from the
> same tape.

No, I have never, ever, ever said anything about that.
>
>> ... You will be hard pressed to sustain more than 30-40
>> MB/sec with even just 2-3 streams running on a single consumer hard
>> drive.  More streams (concurrent backup jobs) will reduce throughput
>> further.  This isn't FUD, this is REALITY.
> My reality is 340MB/s with 10 clients spooling to consumer ssd and 1
> write job despooling to consumer spinning rust. You may be hard pressed
> to do 1/10th of that in your reality but nobody else has to live there.
>
>
What on earth do you not understand about the physical limits of disk 
drives, and how things change when you change the technology or layout 
of the system?  10 sessions spooling to an SSD is completely different 
from writing to a single drive.  You seem to be saying that you're 
despooling to a RAIDZ1 array with compression enabled, that again has 
nothing to do with single disk performance.

Single disk hard drive performance is all that I've been talking about.  
And single disk hard drives can't be read fast enough to feed an LTO 
drive.  If one is designing a backup system that expects multiple 
sessions to be spooling to a disk device that is also going to be 
despooling to a tape device one needs to use something other than a 
single drive - be it a RAID array, an SSD, etc.

WD Red drives have max throughput ranging from 144MB/sec to 175MB/sec 
depending on capacity:

http://wdc.custhelp.com/app/answers/detail/search/1/a_id/9491#

Seagate NAS drives have a max throughput ranging from 149MB/sec to 
180MB/sec depending on capacity:

http://www.seagate.com/www-content/product-content/nas-fam/nas-hdd/en-us/docs/nas-hdd-ds1789-3-1409gb.pdf

Those are the MAXIMUM throughputs the drives can sustain when doing a 
purely sequential workload.  Actual performance will be less. Start 
moving the heads at all while doing I/O and the rates will plummet 
quickly.  One reader and one writer accessing the drive at the same time 
will be lucky to get a combined total of even 50% of that.

And with that I'm fully sick of this conversation.

Bryn

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