Open Stack

[consider moving the thread to OpenStack-Dev]

John raises an important point below that I think deserves attention not
only from SWIFT developers but also from the INFRA team. Testing patches
for specific systems raises the need for testing environments, for
Solaris and ZFS.

INFRA-Team: what's your take on this?

/stef

On 07/18/2012 02:50 PM, John Dickinson wrote:
> Nexenta's LFS patch (https://review.openstack.org/#/c/7524/) has
> languished for a while, and I'd like to address that.
> 
> First, thank you for your patch submission. This patch adds new
> functionality that potentially can allow swift to be deployed in more
> places. The original version of the patch, which you referenced, was
> quite a bit more complex. Thanks for listening to the feedback from
> the reviewers and refactoring out most of the complexity. The current
> state of the patch appears to be much improved. I do hope that the
> patch can be approved and merged into swift.
> 
> However, there are two things which make it more difficult to approve
> this patch: review time and ability to test.
> 
> This patch touches the ring builder, which is a rather complex part
> of swift. To properly review it, it will take two of the core devs at
> least a day each. Unfortunately, putting other "normal" job duties on
> hold for a day or more is very hard to do. This isn't a problem with
> Nexenta or the patch itself; it actually points to a problem with
> swift. We shouldn't have a part of the code so integral to the system
> that requires a full dev day every time it's touched.
> 
> The other issue with approving the patch is testing. Any new feature
> that is merged into swift becomes something that all swift
> contributors must now support and maintain. The maintenance burden is
> lessened (but not eliminated) by any sort of testing that can be
> provided. The LFS patch adds functionality that cannot be well
> tested. At best, we can only test that the patch doesn't break any
> existing functionality. But we have no way to ensure that later
> patches won't break the functionality that this patch provides. Since
> this patch is currently only really useful with Nexenta's separate
> lfs middleware for ZFS, and since there is no testing infrastructure
> set up to test swift on Solaris/ZFS, we cannot offer any sort of
> support or maintenance for the feature this patch provides.
> 
> If Nexenta would like to provide and run some hardware for testing
> purposes, it would go a long way to helping ensure that this feature
> and others like it can be properly added to and maintained in swift.
> If this LFS patch is indeed accepted, it will be Nextenta's
> responsibility to ensure that all future patches in swift do not
> break the LFS functionality. (This applies to the previously merged
> patch for Solaris compatibility, too.)
> 
> 
> 
> --John
> 
> 
> 
> 
> 
> On Jul 16, 2012, at 3:45 PM, Victor Rodionov wrote:
> 
>> Hello
>> 
>> I've submit patch (https://review.openstack.org/#/c/7101/), that
>> help Swift use special features of file system on that it working.
>> 
>> One of the  changes in this patch is for reduce number of network
>> replicas of partition if user use self-repairing mirrored device.
>> For this user should add mirror_copies parameter to each device. By
>> default mirror_copies for all devices is 1, so changes of code
>> don't take any effect for current Swift deployments.  For almost
>> all systems three singleton replicas can be replaced by two
>> mirrored replicas. So if all user devices is mirrored
>> (mirror_copies >= 2), then number of network copies of most
>> partition will be reduced, and then for operation like PUT and POST
>> we will make less request. The definition of mirroring specifically
>> requires the local file system detect the bad replica on its own,
>> such as by calculating checksums of the content, and automatically
>> repairing data defects when discovered.  So if one of devices fail
>> recovery will be done by file system without coping data from other
>> device. This changes was made in ring builder and take effect if
>> mirror_copies > 1, so this code is not danger for current Swift
>> users, but for other users can provide new possibility.
>> 
>> Also this patch add hooks, that can be used for manipulation with
>> file system, when Swift operate with account, container or object
>> files. This hooks used by middleware that is separate project, so
>> if user don't install it this changes will not take effect.
>> 
>> This feature only enabled by customers that have chosen to install
>> the enabling software and turn it on and it is easy to test that
>> this patches have no impact on the generic deployments.
>> 
>> Most of patch code was restructured, most of logic was moved to
>> middleware level and use hooks in Swift code. I create separate
>> project (LFS middleware https://github.com/nexenta/lfs) for now
>> there are only 2 supported file system types (XFS and ZFS) there.
>> Also this middleware provide API for getting file system status
>> information (for example, for ZFS it's current pool status, etc).
>> 
>> Further the Nexenta side-project is not the only local file system
>> that could provide this form of local replication and data
>> protection.Trading off between network replication and local
>> replication is a valid performance decision. Insisting on a fixed
>> amount of network replication without regard to the degree of local
>> protection provided against data loss would effectively bias the
>> system towards network replication only. Why pay for local data
>> protection if you cannot reduce the amount you pay for network
>> replication? This patch enables solutions that widen the range of
>> choices available to users as to how they protect their data.
>> 
>> Thanks, Victor Rodionov 
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> 
> 
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