Il 02/04/2012 23:55, Dan McAllister ha scritto:
From an earlier post:
Of course your hardware configuration can make a difference too.
You mentioned 2x2TB drives. I hope you have them configured as
raid-1 mirrors (software raid should do fine for raid-1). raid-1
will give you a little slower write, but faster read performance.
I'm a little bit of a stickler on RAID assumptions... so I am going to
stick my nose into this (where it probably does not belong).
Indulge me or not -- I've been up-front about what I'm doing here! :-)
There are a *significant *PERFORMANCE differences between hardware and
software RAID -- even RAID-1. There are several criteria to consider
for each technology -- specifically, read performance and write
performance under both normal and degraded conditions, so here goes:
* Normal RAID-1 Read Times:
There is no benefit in software RAID-1 for read-times on good
(healthy) arrays -- whether you read from drive 1 or drive 0 won't
matter, except for the potential overlapping of seek times, which
compared to hardware RAID-1 is minimal. There is only the one I/O
channel on a standard controller, and so you can actually accept
the data (I/O from drive to controller) from only 1 drive at a time.
There /can be /a considerable performance *increase *in using
HARDWARE RAID-1, assuming your controller supports parallel reads
(see below on "/cheap/" RAID cards). On small reads, the increase
can be anywhere from 25-40%, on larger file reads, the increase
can approach a /theoretical /max of 50%! (By increase, I mean
performance increase -- as measured by /decreases /in wait times
from the moment a read request is received to the moment it is
satisfied back to the calling routine).
* Degraded RAID-1 Read Times:
In software RAID-1, a failed drive can essentially bring down a
server as the controller continually tries to access a failing or
failed drive (assuming the drive isn't unrecognizable) and blocks
other I/O as a result. The only fix is to reboot (often in single
user mode) and mark the bad drive as down or removed. MOST
software RAID (e.g. embedded motherboard controllers) cannot (or
more correctly, WILL not) do this automatically -- they just
continue to flail themselves against the failing drive, making the
system response time measured in minutes (vs. milliseconds). Odd
side-note: Linux MD RAID-1 performs MUCH better in the degraded
mode, because it is much faster to mark a bad drive as down, thus
ending the I/O blocks that otherwise cripple the system.
In hardware RAID-1, a failed drive is usually taken offline
/automatically /by the controller (similar to the response time of
the Linux MD software solution, but NOT similar to the cheap or
motherboard RAID solution that tries repeatedly to access a bad
drive) -- so read times essentially revert to JBOD (single disk)
read performance levels. There can be some hiccups while the card
determines the good/bad status of the bad drive, but better
hardware RAID cards do this while continuing to read from the
"good" drive (making a new read request while it "works on" the
bad drive) -- and during this time, read performance can be
impacted, but usually only slightly, and NOT essentially blocking
the entire server behind the I/O waits.... then, once the drive
testing is done and it is marked down, the performance essentially
equals JBOD performance (as with the software solution, once the
bad drive is removed -- logically or physically).
* Normal RAID-1 Write Times:
There is a *write-penalty* in software RAID-1, even on good arrays
-- because all data has to be written twice, and even with NCQ,
the controller has to "talk" to each drive independently (that is,
send the write data to each drive, one drive at a time). NCQ does
offset this somewhat (assuming you enabled it), but not during
heavy load times because there is really only one I/O channel on
the drive controller (even on SATA RAID controllers).
There is usually *_no write-penalty_* in hardware RAID-1, because
the controllers are designed to "talk" to each drive
independently. (NOTE: Some cheaper RAID cards use non-RAID
optimized controllers, in which case the performance is the same
as with the software RAID, except you're not spending the
processing time on the main system, you're using the on-board RAID
processor on the card.) If you're interested in write-performance
improvements, INSIST on a /_*REAL *_/hardware RAID solution!
* Degraded RAID-1 Write Times:
Again, in software RAID-1, a failed drive can essentially bring
down a server as the controller continues to try to write to a
failing or failed drive (assuming the drive isn't unrecognizable).
As with the read function, the only reasonably fast fix is to
reboot in single user mode and mark the bad drive down. You often
cannot do this with the system live because the response times are
practically non-existent due to the I/O blocks on the bad drive.
Also, this may be difficult on a remote server because, even in
single-user mode, if the failed array contains the root partition,
you'll get I/O blocked loading even in single-user mode!
In hardware RAID-1, a failed drive is usually taken offline
entirely automatically, so write times revert to JBOD performance.
There can be some hiccups while the card determines the good/bad
status of the bad drive (as noted earlier), but better hardware
RAID cards do this while continuing to write to the "good" drive.
NOTE: I do make assumptions here -- notably that the "hardware" RAID
solution you purchase is a card that really does implement RAID, vs.
/simulating /RAID (like the software versions do). A $50 RAID card is
not a /REAL /RAID card... most $200+ RAID cards are -- but read the
specs carefully! (This is kind of like the old "real modem" vs.
"win-modem" debates... just as back then, you want the "hardware
accelerated" RAID solution -- which is more expensive, but well worth it!)
To me, the performance issues during the degraded state alone makes
the extra hardware purchase well worth the extra $$$. I personally
like both the Adaptec and AMCC/3-Ware (now LSI) solutions for SATA
RAID arrays. After all, the whole point of the RAID-1 array is to
protect the overall system WHEN A DRIVE FAILS... if all you wanted was
faster performance, use RAID-0 (no protection, just striping!)
One last point -- back in the 1990's, RAID 3 and RAID 5 (and later,
RAID 6) were popular because storage was _/expensive /_(and most RAID
arrays were using *SCSI *drives, which were significantly _more
_expensive than PATA or SATA drives). But times have changed and 2TB
SATA (or even SAS) drives are _cheap and common._ The only RAID
technology that out-PERFORMS a hardware RAID 1 array is a hardware
RAID-10 array (which is a striped mirror)... and I personally only use
those for EXTREMELY large RAID-1 arrays!
OK... so this is the end of my RAID tutorial... I hope you learned
something! :-)
Dan McAllister
IT4SOHO
PS: I learned all of this when I worked as a management consultant to
EMC back in the early 2000's... back when hardware RAID-1 and RAID-5
were at or near the "tipping point" in cost/benefit, so it was often a
"use case" issue as to which technology was more effective for a
particular client. We were often up against "homemade" external RAID
systems or humongous RAID5 arrays from IBM or Hitachi -- we were early
adopters of RAID-1 for performance results! Of course, it didn't hurt
that RAID-1 requires more physical drives! :-)
World is grey sometimes, between the white of HW raid and the black of
SW raid.
I am a fan of hardware raid, and we use at the most HP SmartArray cards,
as we feel they are very reliable and fast.
But, usage of ZFS in last installations put us in front of a new age of
thinking: ZFS reccomends to disable hardware raid and use JBOS instead.
ZFS will care about RAID. So, willing to use this new FS, we have no
other choice than have HW raid on boot disks (with UFS), and JBOD on
other disks (with ZFS).
At the same time, while we are currently using RAID 10 and RAID 5, I
still see very serious reasons to use RAID6 in some environments.
Disks are generally cheap, but serious disks are expensive, despite of
the huge size. So I don't blame is someone is putting 12 x 2TB disks in
a RAID6 configuration for slow speed or dedicated storage (like backups
or seldom used archives). 12 disks in RAID10 means 5 used disks (5 are
mirrored, 2 are hot spairs). 12 disks in RAID6 means 9 used disks (2 for
RAID6, one for hot spare). Difference in used storage is a lot.
I agree RAID10 is a lot faster, but in some environments this could not
be important, while price and reliability could be more important.
Regards,
Tonino
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