Hello,
2015-01-23 21:36 GMT+01:00 Josh Fisher <jfis...@pvct.com>:
>
> On 1/23/2015 2:33 PM, Heitor Faria wrote:
> >> On 01/23/2015 12:22 PM, Radosław Korzeniewski wrote:
> >>
> >>> If you are using a vchanger then I guess it is a disk only backup, so
> why
> >>> do you spool data? It is not required but making you backup slow down
> at
> >>> least 2 times then standard job.
> >> It's 2 times slower if your spool/despool is strictly sequential and the
> >> same size pipes go in and out. If you have 10 clients spooling over a
> >> 1Gb/s link, that's roughly 10MB/s/client. Iostat clocks ext4/basic sata
> >> drives at around 110MB/s write speed, so despooling is 10 times faster.
> >> If only the clients could keep the pipe full, it could be ~10 times
> >> faster than the "standard" job. In a purely hypothetical perfectly
> >> spherical world of uniform density, obviously, but still: no, 2x slow
> >> down is not how it really works.
> > Even though, IMHO, spooling disks backup is just "muda" (Japanese Term):
> http://en.wikipedia.org/wiki/Muda_(Japanese_term)
>
> No. It is not wasteful if it serves a purpose. The goal is to get the
> client's part in the job done faster.
You can do a client's part of the backup job faster only when a whole job
will fit in spooling area. But it will not make a backup job to finish
earlier.
If you have enough space you should consider implementing D2D2T/D strategy.
> Fast storage is expensive. A
> shared spool area on fast disk gets the clients part done faster,
Only when a whole job will fit into this area. If not then a particular job
needs to be stopped for despooling. In the overall the job speed will be as
fast as your slow storage. You can make a calculations to check if I'm
right or not. I've made this calculations for Dimitri in my other email.
> and
> then the SD can de-spool to slower (and cheaper) volume file storage
> without affecting the client.
Affecting a client is always a matter of read speed and time. If your
backup job saturate a IO subsystem, especially disks and its interface or
network interface then it is a problem for any service running on this
server. For this purpose there is a network bandwidth limitation feature.
It can limit utilization level during backup. On the other hand when you
backup with snapshots (with limited snapshot space) or offline database
backups then you want to finish your backup as soon as possible saturating
all your available resources on the client side.
A fast spooling storage will not help you as during despooling your backup
is still running holding a database in offline state or snapshot, etc.
> Using spooling for disk volumes is exactly
> the same reasoning as using it for tape volumes.
>
No. No, and again No.
Why anyone wants to use data spooling for tapes? Because a tape drive
require minimal throughput to avoid shoe-shine of the tape during write.
When your are unable to provide a minimal throughput to the tape drive
directly from the client you need to buffer (spool) data on the local
storage which should be fast enough to provide optimal write speed to the
tape drive. So, you have: client throughput < tape drive minimal
throughput. And this is a purpose of data spooling.
For disk based storage it is not required to provide a minimal throughput.
best regards
--
Radosław Korzeniewski
rados...@korzeniewski.net
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