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From:Karen Abgarian Date:March 19 2011 12:22am
Subject:Re: mysql using aio/raw device on linux
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Hi... 

If we are to compare MySQL/binlog with Oracle's redo log, there are some known
differences.   First of all, the (somewhat) equivalent structure in Oracle for the
MySQL's binary log is not the redo log the log writer is writing to.   It is the archived
log.  The function of those is also different.  The redo log is needed for crash recovery.
 The archived log is needed for media recovery.  

Second, In Oracle, transactions always have to wait for commits.  Taking that as the
starting point, the optimizations for writing the redo are put in place.   The approach
is therefore "we have to write, so let us optimize it".  As can be seen, it does not make
sense to optimize redo writing unless we are targeting zero transaction loss.   There is a
large difference between zero and "some" in this context.   

So, if we ever wanted to optimize the mysql's writing to binary logs, it is unclear what
we would do that for.   It cannot be protection against server crashes, because that
could be resolved by using InnoDB or any other transactional storage engine.  It is not
protection against media failures, because a loss of the binary log because of media
failures is equivalent to losing the archived log in Oracle for the same reasons.  

The only thing that is somewhat impacted is replication on host crashes.  However, if we
wanted to be precise on this, we would notice that similarly to losing the transactions
in the OS cache, we could also lose updates to the MySQL tables at the same time.   Which
means that on server crashes the primary servers should be abandoned and the service be
switched to replica.   If this is done, it does not matter that some transactions are
lost (as it does not when the same thing is done in Oracle, during primary media failures
and provided it is not configured to not lose data between primary and standby). 


Tx
Karen.



On Mar 17, 2011, at 7:15 PM, Zhu,Chao wrote:

> Thanks Guys;
> The reason I was seeking RAW/AIO, is mostly about non-blocking write;
> Which i mean:
>    Even though single write is not faster on RAW, if it supports raw and
> Asynch IO write, then MySQL can continue to submit write request to disk
> without waiting for the previous write to complete, and then submit a second
> write request;
> In this case,  the commit(write throughput) can be enhanced greatly, without
> blocking /keeping user wait;  In our current test, we are using SAN with
> huge cache and each single write only takes 0.3ms(yes very fast, close to
> ramdisk i guess); But the sequential/blocking fsync call is the bottleneck:
> and it can't  be parallized;
> 
> That's the reason i was seeking for such option;
> 
> I was an oracle DBA before and oracle has such kind of capability(aio write)
> so lgwr can have very high throughput(tens of thousands of commit per
> second, and it does group commit);
> 
> Sample Trace in Unix/Oracle lgwr:
> /1:     semtimedop(105, 0xFFFFFFFF7FFFC914, 1, 0xFFFFFFFF7FFFC900) = 0
> /1:     kaio(AIOWRITE, 261, 0x390D3CE00, 8704, 0x0F5FB0007BB2B218) = 0
> /1:     kaio(AIOWRITE, 261, 0x390C80000, 253952, 0x0F5FD2007BB2B4A8) = 0
> /1:     kaio(AIOWRITE, 261, 0x390D60400, 211456, 0x0F63B2007BB2B738) = 0
> /1:     kaio(AIOWRITE, 261, 0x390E8EC00, 182272, 0x0F66EC007BB2B9C8) = 0
> /1:     kaio(AIOWRITE, 261, 0x390F10A00, 230912, 0x0F69B4007BB2BC58) = 0
> /1:     kaio(AIOWRITE, 261, 0x391024A00, 91648, 0x0F6D3A007BB2BEE8) = 0
> 
> Thx
> 
> 
> On Fri, Mar 18, 2011 at 6:00 AM, Karen Abgarian <abvk@stripped> wrote:
> 
>> Hi,
>> 
>> For the actual question, I agree with the points Johan mentioned.   MySQL,
>> to my knowledge, does not have an option to use raw devices for binary logs.
>> Even if it had it, it would not have the benefits Chao is seeking.   There
>> is indeed a tradeoff between losing transactions and performance.   If the
>> goal is performance, the raw device would be slower since  every write would
>> have to actually complete, instead of leaving the block in the OS cache.
>> The best is probably achieved by the battery backed cache: the server could
>> be configured to not lose transactions and at the same time perform the work
>> fast.
>> 
>> For the question of tweaking the sync_binlog, I find difficult to use
>> values other than 0 and 1.   With 0, it just ignores fsyncs, and the amount
>> of transactions lost is at the mercy of OS cache.  With 1, all transactions
>> will always be on disk before returning to the user.  I cannot make sense
>> out of the doco's remark about that this would lose 'at most one
>> transaction' and I assume it is a mistake.
>> 
>> With the value of 10, say, however, what I expect to happen, is the server
>> will attempt to do fsync every 10 statements.   Say 10 transactions are in
>> the binary log buffer, and the server does an fsync.  What is to happen with
>> the other transactions that keep coming?  If they commit in memory and
>> return, the statement that sync_binlog syncs every 10 transactions is false.
>>  If they wait, the wait would be as large as the wait for the disk write
>> and the result is that all transactions will be waiting for disk writes.
>> 
>> If somebody can shed more light on this, I would like to hear it.
>> 
>> Tx
>> Karen.
>> 
>> 
>> On Mar 17, 2011, at 12:14 AM, Johan De Meersman wrote:
>> 
>>> ----- Original Message -----
>>>> From: "Chao Zhu" <zhuchao@stripped>
>>>> 
>>>>  One Q: Can mysql binlog use raw device on Linux?
>>> 
>>> Mmm, good question. Don't really know; but I'm not convinced you'll get
>> huge benefits from it, either. Modern filesystems tend to perform pretty
>> close to raw throughput.
>>> 
>>> From a just-thinking-it-through point of view, I'd guess no - mysqld
>> never seems to open binlogs for append, it always opens a new one. This may
>> have something to do with the way replication works; not to mention the
>> question of what'll happen if the log is full - it's not a circular buffer.
>>> 
>>>> Can we use asynch IO for binlog writing? sequential non-qio fsync is
>> slowing our throughput...
>>> 
>>> Mmm... Theoretically, yes, you could use an async device (even nfs over
>> UDP if you're so inclined) but async means that you're going to be losing
>> some transactions if the server crashes.
>>> 
>>> You can also tweak
>>
> http://dev.mysql.com/doc/refman/5.0/en/replication-options-binary-log.html#sysvar_sync_binlog-
> basically, this controls how often the binlog fsyncs. Same caveat applies,
>> obviously: set this to ten, and you'll have ten times less fsyncs, but you
>> risk losing ten transactions in a crash.
>>> 
>>> If your binlogs are async, then you also risk having slaves out of sync
>> if your master crashes.
>>> 
>>> 
>>> Personally, if your binlogs are slowing you down, I would recommend
>> putting them on faster storage. Multiple small, fast disks in RAID10 are
>> going to be very fast, or you could invest in solid state disks - not all
>> that expensive anymore, really. Maybe even just a RAM disk - you'll lose
>> data when the machine crashes (and need an initscript for save/load of the
>> data on that disk), but not if just the mysqld crashes.
>>> 
>>> 
>>> Weigh the benefits of each option very, very carefully against the risk
>> of losing data before you go through with this.
>>> 
>>> 
>>> --
>>> Bier met grenadyn
>>> Is als mosterd by den wyn
>>> Sy die't drinkt, is eene kwezel
>>> Hy die't drinkt, is ras een ezel
>>> 
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>>> 
>> 
>> 
>> --
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>> 
>> 
> 
> 
> -- 
> Regards
> Zhu Chao

Thread
mysql using aio/raw device on linuxChao Zhu16 Mar
  • Re: mysql using aio/raw device on linuxJohan De Meersman17 Mar
    • Re: mysql using aio/raw device on linuxKaren Abgarian17 Mar
      • Re: mysql using aio/raw device on linuxChao Zhu18 Mar
        • Re: mysql using aio/raw device on linuxClaudio Nanni18 Mar
      • Re: mysql using aio/raw device on linuxChao Zhu18 Mar
        • Re: mysql using aio/raw device on linuxKaren Abgarian19 Mar