Mirroring and different sector sizes? BAD IDEA!

  • Theo Ekelmans

    SSCarpal Tunnel

    Points: 4482

    Comments posted to this topic are about the item Mirroring and different sector sizes? BAD IDEA!

  • T2000

    SSC Eights!

    Points: 894

    Good & well written article Theo. Useful to know. Thanks

    Rgds,

    T.

    .be

  • Nakul Vachhrajani

    SSChampion

    Points: 10187

    Very well written article. Thank-you very much for the same.

    It's difficult to keep cool and look for the root cause when such problems arise. I therefore, appreciate your dedication to the issue. I am sure this will help out a lot of the members of the community.

    Thanks & Regards,
    Nakul Vachhrajani.
    http://nakulvachhrajani.com
    Be courteous. Drive responsibly.

    Follow me on
    Twitter: @sqltwins

  • Wayne Evans-440401

    SSC Eights!

    Points: 957

    Thank you. This is bound to bite me in the future.

    Bookmarked


    Wayne

    Did you get access denied? Great the security works.

  • rfr.ferrari

    SSCertifiable

    Points: 6879

    Thanks Theo for share your experiences with us!!!!

    This article is really very important


    [font="Times New Roman"]rfr.ferrari[/font]
    DBA - SQL Server 2008
    MCITP | MCTS

    remember is live or suffer twice!
    the period you fastest growing is the most difficult period of your life!
  • vdrgcfjhyuhjcyurnsxjry

    Valued Member

    Points: 61

    Excellent item. This is a fine example of how to write a technical article.

    Thanks.

  • OzYbOi d(-_-)b

    Hall of Fame

    Points: 3994

    tks for lesson learned without all the pain you endured! cheers

  • henderson1134

    SSC Rookie

    Points: 33

    The title of the article is about Mirroring with different sector sizes but the content of the article is about disk misalignment. Very helpful info but maybe a title change?

    I have several clients who have bought new SANs that go with a 4k sector sized LUN but keep the old SAN as their mirror failover. In some cases they won't or can't rebuild the old SAN's LUNs to be 4k sector based so they get warnings in the SQL Server logs about the tail of the log having to be re-written. It will slow down performance if the mirroring process has to take each 4k chunk and remap it back to 512B and then process each of the 512B chunks.. but its effects are dependent. One client I have has two HP DL980's running 160 logical cores doing all OLTP and they're mirroring is working fine enough in this setup. It spams their error logs but that's the price of not upgrading the old SAN.

    Disk alignment, as you have shown, causes more IO to be required to do the same task. For a read it will be 2x. It has to grab both physical sectors that the logical sector is on in order to reconstitute the logical sector. For writes it can be worse. It's called a Read Modify Write. It has to read both physical sectors, then modify the parts that changed on both physical sectors, then write both physical sectors to storage. So there's 4x the IO's plus two 'update' transactions in the storage tier.

  • Andy Hogg

    SSCarpal Tunnel

    Points: 4525

    Great article and a very clear and detailed explanation - thank you very much.

    Have just seen this exact same problem with a 4 replica AlwaysOn Availability Group.

    Sure enough the disk sector size on one of the replica servers is different to that on the other three.

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