Re: is mdadm RAID1 disk full sync

[lingli tang <tanglingli001@gmail.com>]

Thanks very much.
I will try DRBD later
But I want to figure this out.

I have export disk using tgtd and load disk on another server using
iscsiadm with infiniband  of iser protocol.
Does ISCSI/Iser have any cache on it.


2015-03-22 15:28 GMT+08:00 Adam Goryachev <mailinglists@websitemanagers.com.au>:
>
>
> On 22/03/2015 16:00, lingli tang wrote:
>>
>> Thanks for reply.
>>
>> I have create a raid1 with two fusion io PCIe flash disk:
>> mdadm --create /dev/md/master --name=master --level=1 --raid-devices=2
>> /dev/fioa2 /dev/mapper/mpathc
>> /dev/fioa2 is local disk on server A and /dev/mapper/mpathc is a iscsi
>> load disk export from server B.
>>
>> After that we mkfs.ext4 on /dev/md/master and mount with 'sync' option on
>> /data1
>> and we will run mysql binlog on it.
>> In order to avoid data loss  of mysql binlog we have set
>> sync_binlog=1. so every sql commit will call fsync() to flush to disk.
>>
>> according to your description. if we reboot the server A, the two disk
>> data on different server will be the same.
>> but after the server A restarted, we assemble the two disk on two
>> server, data is different on the two server, disk on server B lost
>> more than one sql commit.
>>
>> I have checked it with strace 'mysqld' on Server A.
>> I found a sql commit and fsync() on binlog file handle on server A but
>> this sql can not find in assembled disk on server B.
>>
>> I also test it with two SAS disk, Server B still has more than one sql
>> commit lost.
>
> Sounds like you might be better using something like DRBD (www.drbd.org)
> which has different modes, one of which will do what you are asking (not
> respond until both systems have confirmed the data is written to the local
> disk).
>
> In your current case, even if md is correctly writing to both underlying
> 'devices' you have multiple layers under one of the devices, so you should
> confirm that *all* of those layers are properly passing through the data
> without any caching/etc.
>
> Regards,
> Adam