On 05/05/2012 08:02 AM, Anand Avati wrote:
On Wed, May 2, 2012 at 3:55 AM, Xavier
Hernandez <xhernandez@xxxxxxxxxx>
wrote:
Hello,
I'm wondering if there are any requisites that translators
must satisfy to work correctly inside glusterfs.
In particular I need to know two things:
1. Are translators required to respect the order in which they
receive the requests ?
This is specially important in translators such as
performance/io-threads or caching ones. It seems that these
translators can reorder requests. If this is the case, is
there any way to force some order between requests ? can
inodelk/entrylk be used to force the order ?
Translators are not expected to maintain ordering of
requests. The only translator which takes care of ordering
calls is write-behind. After acknowledging back write requests
it has to make sure future requests see the true "effect" as
though the previous write actually completed. To that end, it
queues future "dependent" requests till the write
acknowledgement is received from the server.
inodelk/entrylk calls help achieve synchronization among
clients (by getting into a critical section) - just like a
mutex. It is an arbitrator. It does not help for ordering of
two calls. If one call must strictly complete after another
call from your translator's point of view (i.e, if it has such
a requirement), then the latter call's STACK_WIND must happen
in the callback of the former's STACK_UNWIND path. There are
no guarantees maintained by the system to ensure that a second
STACK_WIND issued right after a first STACK_WIND will complete
and callback in the same order. Write-behind does all its
ordering gimmicks only because it STACK_UNWINDs a write call
prematurely and therefore must maintain the causal effects by
means of queueing new requests behind the downcall towards the
server.
Good to know
2. Are translators required to propagate callback arguments
even if the result of the operation is an error ? and if an
internal translator error occurs ?
Usually no. If op_ret is -1, only op_errno is expected to
be a usable value. Rest of the callback parameters are junk.
When a translator has multiple subvolumes, I've seen that some
arguments, such as xdata, are replaced with NULL. This can be
understood, but are regular translators (those that only have
one subvolume) allowed to do that or must they preserve the
value of xdata, even in the case of an internal error ?
It is best to preserve the arguments unless you know
specifically what you are doing. In case of error, all the
non-op_{ret,errno} arguments are typically junk, including
xdata.
If this is not a requisite, xdata loses it's function of
delivering back extra information.
Can you explain? Are you seeing a use case for having a
valid xdata in the callback even with op_ret == -1?
As a part of a translator that I'm developing that works with
multiple subvolumes, I need to implement some healing support to
mantain data coherency (similar to AFR). After some thought, I
decided that it could be advantageous to use a dedicated healing
translator located near the bottom of the translators stack on the
servers. This translator won't work by itself, it only adds support
to be used by a higher level translator, which have to manage the
logic of the healing and decide when a node needs to be healed.
To do this, sometimes I need to return an error because an operation
cannot be completed due to some condition related with healing
itself (not with the underlying storage). However I need to send
some specific healing information to let the upper translator know
how it has to handle the detected condition.
I cannot send a success answer because intermediate translators
could take the fake data as valid and they could begin to operate
incorrectly or even create inconsistencies. The other alternative is
to use op_errno to encode the extra data, but this will also be
difficult, even impossible in some cases, due to the amount of data
and the complexity to combine it with an error code without mislead
intermediate translators with strange or invalid error codes.
I talked with John Mark about this translator and he suggested me to
discuss it over the list. Therefore I'll initiate another thread to
expose in more detail how it works and I would appreciate very much
your opinion, and that of the other developers, about it. Especially
if it can really be faster/safer that other solutions or not, or if
you find any problem or have any suggestion to improve it. I think
it could also be used by AFR and any future translator that may need
some healing capabilities.
Thank you very much,
Xavi
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