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List:       drbd-user
Subject:    Re: [DRBD-user] writer order on secondary site
From:       Mia Lueng <xiaozunvlg () gmail ! com>
Date:       2015-12-27 5:26:15
Message-ID: CACk5js07x=DAAzQ_PxsnCycCg-1HOKwruNdiFnn=dXikEmhyQg () mail ! gmail ! com
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How does epoch works? I have examined the source code.  when primary
recieves a bio with bi_size=0 or  bio num > MAX_EPOCH_SIZE or idle
time exceeds limit , there will be a P_BARRIER packet triggered?

2015-12-24 23:49 GMT+08:00 Lars Ellenberg <lars.ellenberg@linbit.com>:
> On Wed, Dec 23, 2015 at 12:11:51AM +0800, Mia Lueng wrote:
>> Hi:
>> I'm just wondering how secondary handle the write ordering when a same
>> block is written twice on primary.
>>
>> Application submits these updates: X, Y, Z.
>> They may or may not be to the same block.
>> If they are to the same block, then the application, file system
>> or other layer already makes sure (or at least is supposed to do that),
>> that the first update will finish before the second is submitted.
>>
>> These updates are replicated to the peer.
>>
>> When X,Y are to the same block, how secondary site make sure that the
>> first update will finish before the second is summited?
>
> In synchronous mode, this is trivially obvious.
> In asynchronous mode, we have DRBD barriers, and epochs.
>
> Basic idea is: whenever we complete something to upper layers,
> we bump an atomic epoch counter.
> Anything sumitted within one epoch cannot possibly depend on
> something else within that same epoch.
> So within such an epoch, reordering is allowed.
> "on the wire", epochs are separated by "DRBD barriers".
> Receiving side uses drain/flush/barrier (depending on configuration)
> to make sure anything from a previous epoch is on stable storage
> before starting to submit for the next epoch.
>
> --
> : Lars Ellenberg
> : http://www.LINBIT.com | Your Way to High Availability
> : DRBD, Linux-HA  and  Pacemaker support and consulting
>
> DRBD ® and LINBIT ® are registered trademarks of LINBIT, Austria.
> __
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