Writes Taking Longer than Reads - sQLIO

Posted on 2011-10-29
Last Modified: 2012-05-12
Hi there,

We are in the process of installing a Dell Equallogic SAN Storage solution.  We have virtualised our servers and put them on the hosts sitting above the Dell EQL.

Having ran SQLIO to test 8, 16, 32, 64 and 256Kb of data we have discovered the following odd behaviour:
Reads are happening at 110MBps whilst Writes are 45MBps.  We are using 1GBps ethernet (so 110 is expected bandwidth limit).

Has anyone seen and/or does anyone have an explanation for this behaviour?

Many thanks,


Question by:Evolution-Recruitment
    LVL 82

    Expert Comment

    by:Dave Baldwin
    That is common.  Writing is a more complicated process than reading, especially for SQL.  When reading, the data just has to be located and returned.  When writing, the proper place to put it must be found, it has to be written, and any indexes have to be updated also.  Writing takes two or three operations while reading can take only one.

    Author Comment

    I would agree but we have 24 disks in the SAN and other servers are getting the same write bottleneck at the same time. If it was the disks i would expect the write bandwidth to reduce when 2 servers are writing.
    LVL 82

    Accepted Solution

    I don't think it's the disks although disks do 'read ahead' so that large blocks can sometimes return faster than writing the blocks.  I think the problem is with the SQL process.  You can't just write to the database files.  You have to read any info you're going to change or add to first to find out where to put the new or updated info.  And it's probably at least two reads and two writes including the index and data.
    LVL 75

    Expert Comment

    by:Anthony Perkins
    I just want to make sure I understand:  You were expecting the writes to be faster than the reads in your specific case?  

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