IDS Forum

Re: SSD RAID selection

Posted By: Art Kagel
Date: Thursday, 11 June 2015, at 12:30 p.m.

In Response To: Re: SSD RAID selection (KAMRAN HAQ)

I'll back up what Lester said. The problem with the server possibly going
offline when there is a RAID5 drive failure is twofold:

1. Until a replacement drive is assigned to be used to rebuild the down

drive reads will be delayed while the SAN has to read all of the remaining

drives to recreate data that resided on the failed drive if such data is

requested. This process can be slow. Sometimes Informix will decide that

the drive has failed if the reads take too long. The performance impact

until recovery begins can be 25% or more for a five drive array.

2. Similarly once a replacement drive has been assigned, reading and

writing will be slowed by as much as 80% in other words will be about five

times slower than the full array. In this case it is more likely that

Informix will determine that an IO has timed out and mark the offending

chunks offline.

I have seen both of these scenarios happen with EMC and others' arrays. Of
course, since IO is so slow during recovery, recovery takes far longer for
a lost RAID5 drive than it does for a lost RAID10 drive putting your system
more at risk for a second drive failure before recovery completes which
would be catastrophic and unrecoverable. Any problem that causes a drive
failure is HIGHLY LIKELY to affect multiple drives. Why? Because if one
drive has reached its physical lifetime limit and failed due to old age,
then all of the other drives installed at the same time are likely in the
same state and will also soon fail. If the drive has failed early then
this is likely caused by a physical or firmware defect in the drive and any
other drives from the same manufacturer's production lot will also be on
the verge of experiencing an identical failure. In a RAID5 array, with
five or more drives each dependent on the other, it is nearly impossible to
have all of the drives coming from different production lots! In a RAID10
array you only have to worry that each member of a RAID1 pair is from a
different lot and if you carefully mix in three or more production lots
then the probability of a multiple drive failure causing catastrophic data
loss is near zero whereas for RAID5 it is close to 100% in my opinion (and
I'm not alone).

As to EMC's comment that bit loss is not an issue, I will point to a study
done by the physicists at CERN recently that proved that it is indeed a
real issue for both magnetic drives and SSD drives.

Just check out the comments on the membership list on the BAARF site:

www.baarf.com/BAARF_members_sql.php
you will see many instances of folks who, like me, have experienced RAID5
data loss.

Art

Art S. Kagel, President and Principal Consultant
ASK Database Management
www.askdbmgt.com

Blog: http://informix-myview.blogspot.com/

Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on the IIUG, nor any other organization with which I am
associated either explicitly, implicitly, or by inference. Neither do
those opinions reflect those of other individuals affiliated with any
entity with which I am affiliated nor those of the entities themselves.

On Thu, Jun 11, 2015 at 11:59 AM, KAMRAN HAQ <khaq@i2cinc.com> wrote:

> Thanks for the guidance.
> Can you please tell why would the database go down. For example if
> database is
> residing a (4+1) RAID 5 and one of the drives fails. Let's the drive to
> replace gets complete in X unit of time. is this that time cause some
> issue or
> anything else?
>
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

--089e0116197a7f919d0518408052

Messages In This Thread

• SSD RAID selection
  KAMRAN HAQ -- Wednesday, 10 June 2015, at 1:03 p.m.
  • Re: SSD RAID selection
    Art Kagel -- Wednesday, 10 June 2015, at 1:16 p.m.
  • Re: SSD RAID selection
    Lester Knutsen -- Wednesday, 10 June 2015, at 4:19 p.m.
    • Re: SSD RAID selection
      KAMRAN HAQ -- Thursday, 11 June 2015, at 11:59 a.m.
      • Re: SSD RAID selection
        Art Kagel -- Thursday, 11 June 2015, at 12:30 p.m.
      • Re: SSD RAID selection
        Lester Knutsen -- Thursday, 11 June 2015, at 10:53 p.m.

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