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From Edison Su <Edison...@citrix.com>
Subject [DISCUSS] NFS cache storage issue on object_store
Date Mon, 03 Jun 2013 23:17:18 GMT
Let's start a new thread about NFS cache storage issues on object_store.
First, I'll go through how NFS storage works on master branch, then how it works on object_store
branch, then let's talk about the "issues".

0.       Why we need NFS secondary storage? Nfs secondary storage is used as a place to store
templates/snapshots etc, it's zone wide, and it's widely supported by most of hypervisors(except
HyperV). NFS storage exists in CloudStack since 1.x. With the rising of object storage, like
S3/Swift, CloudStack adds the support of Swift in 3.x, and S3 in 4.0. You may wonder, if S3/Swift
is used as the place to store templates/snapshots, then why we still need NFS secondary storage?

There are two reasons for that:

a.       CloudStack storage code is tightly coupled with NFS secondary storage, so when adding
Swift/S3 support, it's likely to take shortcut, leave NFS secondary storage as it is.

b.      Certain hypervisors, and certain storage related operations, can not directly operate
on object storage.
Examples:

b.1 When backing up snapshot(the snapshot taken from xenserver hypervisor) from primary storage
to S3 in xenserver

If there are snapshot chains on the volume, and if we want to coalesce the snapshot chains
into a new disk, then copy it to S3, we either, coalesce the snapshot chains on primary storage,
or on an extra storage repository (SR) that supported by Xenserver.

If we coalesce it on primary storage, then may blow up the primary storage, as the coalesced
new disk may need a lot of space(thinking about, the new disk will contain all the content
in from leaf snapshot, all the way up to base template), but the primary storage is not planned
to this operation(cloudstack mgt server is unaware of this operation, the mgt server may think
the primary storage still has enough space to create volumes).

While xenserver doesn't have API to coalesce snapshots directly to S3, so we have to use other
storages that supported by Xenserver, that's why the NFS storage is used during snapshot backup.
So what we did is that first call xenserver api to coalesce the snapshot to NFS storage, then
copy the newly created file into S3. This is what we did on both master branch and object_store
branch.
                               b.2 When create volume from snapshot if the snapshot is stored
on S3.
                                                 If the snapshot is a delta snapshot, we need
to coalesce them into a new volume. We can't coalesce snapshots directly on S3, AFAIK, so
we have to download the snapshot and its parents into somewhere, then coalesce them with xenserver's
tools. Again, there are two options, one is to download all the snapshots into primary storage,
or download them into NFS storage:
                                                If we download all the snapshots into primary
storage directly from S3, then first we need find a way import snapshot from S3 into Primary
storage(if primary storage is a block device, then need extra care) and then coalesce them.
If we go this way, need to find a primary storage with enough space, and even worse, if the
primary storage is not zone-wide, then later on, we may need to copy the volume from one primary
storage to another, which is time consuming.
                                                If we download all the snapshots into NFS
storage from S3, then coalesce them, and then copy the volume to primary storage. As the NFS
storage is zone wide, so, you can copy the volume into whatever primary storage, without extra
copy. This is what we did in master branch and object_store branch.
                              b.3, some hypervisors, or some storages do not support directly
import template into primary storage from a URL. For example, if Ceph is used as primary storage,
when import a template into RBD, need transform a Qcow2 image into RAW disk, then into RBD
format 2. In order to transform an image from Qcow2 image into RAW disk, you need extra file
system, either a local file system(this is what other stack does, which is not scalable to
me), or a NFS storage(this is what can be done on both master and object_store). Or one can
modify hypervisor or storage to support directly import template from S3 into RBD. Here is
the link(http://www.mail-archive.com/ceph-devel@vger.kernel.org/msg14411.html), that Wido
posted.
                 Anyway, there are so many combination of hypervisors and storages: for some
hypervisors with zone wide file system based storage(e.g. KVM + gluster/NFS as primary storage),
you don't need extra nfs storage. Also if you are using VMware or HyperV, which can import
template from a URL, regardless which storage your are using, then you don't need extra NFS
storage. While if you are using xenserver, in order to create volume from delta snapshot,
you will need a NFS storage, or if you are using KVM + Ceph, you also may need a NFS storage.
                Due to above reasons, NFS cache storage is need in certain cases if S3 is
used as secondary storage. The combination of hypervisors and storages are quite complicated,
to use cache storage or not, should be case by case. But as long as cloudstack provides a
framework, gives people the choice to enable/disable cache storage on their own, then I think
the framework is  good enough.


1.       Then let's talk about how NFS storage works on master branch, with or without S3.
If S3 is not used, here is the how NFS storage is used:

1.1   Register a template/ISO: cloudstack downloads the template/ISO into NFS storage.

1.2   Backup snapshot: cloudstack sends a command to xenserver hypervisor, issue vdi.copy
command copy the snapshot to NFS, for kvm, directly use "cp" or "qemu-img convert" to copy
the snapshot into NFS storage.

1.3   Create volume from snapshot: If the snapshot is a delta snapshot, coalesce them on NFS
storage, then vdi.copy it from NFS to primary storage. If it's KVM, use "cp" or "qemu-img
convert" to copy the snapshot from NFS storage to primary storage.


               If S3 is used:

1.4   Register a template/ISO: download the template/ISO into NFS storage first, then there
is background thread, which can upload the template/ISO from NFS storage into S3 regularly.
The template is in Ready state, only means the template is stored on NFS storage, but admin
doesn't know the template is stored on the S3 or not. Even worse, if there are multiple zones,
cloudstack will copy the template from one zone wide NFS storage into another NFS storage
in another zone, while there is already has a region wide S3 available. As the template is
not directly uploaded to S3 when registering a template, it will take several copy in order
to spread the template into a region wide.

1.5   Backup snapshot: cloudstack sends a command to xenserver hypervisor, copy the snapshot
to NFS storage, then immediately, upload the snapshot from NFS storage into S3. The snapshot
is in Backedup state, not only means the snapshot is in  NFS storage, but also means it's
stored on S3.

1.6   Create volume from snapshot: download the snapshot  and it's parent snapshots from S3
into NFS storage, then coalesce and vdi.copy the volume from NFS to primary storage.



2.       Then let's talk about how it works on object_store:
If S3 is not used, there is ZERO change from master branch. How the NFS secondary storage
works before, is the same on object_store.
If S3 is used, and NFS cache storage used also(which is by default):
   2.1 Register a template/ISO: the template/ISO are directly uploaded to S3, there is no
extra copy to NFS storage. When the template is in "Ready" state, means the template is stored
on S3.                  It implies that: the template is immediately available in the region
as soon as it's in Ready State. And admin can clearly knows the status of template on S3,
what's percentage of the uploading, is it failed or succeed? Also if register template failed
for some reason, admin can issue the register template command again. I would say the change
of how to register template into S3 is far better than what we did on master branch.
   2.2 Backup snapshot: it's same as master branch, sends a command to xenserver host, copy
the snapshot into NFS, then upload to S3.
   2.3 Create volume from snapshot: it's the same as master branch, download snapshot and
it's parent snaphots from S3 into NFS, then copy it from NFS to primary storage.
>From above few typical usage cases, you may understand how S3 and NFS cache storage is
used, and what's difference between object_store branch and master branch: basically, we only
change the way how to register a template, nothing else.
If S3 is used, and no NFS cache storage is used(it's possible, depends on which datamotion
strategy is used):
    2.4 Register a template/ISO: it's the same as 2.1
    2.5 Backup snapshot: export the snapshot from primary storage into S3 directly
    2.6 Create volume from snapshot: download snapshots from S3 into primary storage directly,
then coalesce and create volume from it.

          Hope above explanation will tell the truth how the system works on object_store,
and clarify the misconception/misunderstanding  about object_store branch. Even the change
is huge, we still maintain the back compatibility. If you don't want to use S3, only want
to existing NFS storage, it's definitely OK, it works the same as before. If you want to use
S3, we provide a better S3 implementation when registering template/ISO. If you want to use
S3 without NFS storage, that's also definitely OK,  the framework is quite flexible to accommodate
different solutions.

Ok, let's talk  about the NFS storage cache issues.
The issue about NFS cache storage is discussed in several threads, back and forth. All in
all, the NFs cache storage is only one usage case out of three usage cases supported by object_store
branch. It's not something that if it has issue, then everything doesn't work.
In above 2.2 and 2.3, it shows how the NFS cache storage is involved during snapshot related
operations. The complains about there is no aging policy, no capacity planner for NFS cache
storage, is happened when download a snapshot from S3 into NFS, or copy a snapshot from primary
storage into NFS, or download template from S3 into NFS. Yes, it's an issue, the NFS cache
storage can be used out, if there is no capacity planner, and no aging out policy. But can
it be fixed? Is it a design issue?
Let's talk the code: Here is the code related to NFS cache storage, not much, only one class
depends on NFS cache storage: https://git-wip-us.apache.org/repos/asf?p=cloudstack.git;a=blob;f=engine/storage/datamotion/src/org/apache/cloudstack/storage/motion/AncientDataMotionStrategy.java;h=a01d2d30139f70ad8c907b6d6bc9759d47dcc2d6;hb=refs/heads/object_store
Take copyVolumeFromSnapshot as example, which will be called when create Volume from snapshot,
if first calls cacheSnapshotChain, which will call cacheMgr.createCacheObject to download
the snapshot into NFs cache storage. StorageCacheManagerImpl-> createCacheObject is the
only place to create objects on NFs cache storage, the code is at https://git-wip-us.apache.org/repos/asf?p=cloudstack.git;a=blob;f=engine/storage/cache/src/org/apache/cloudstack/storage/cache/manager/StorageCacheManagerImpl.java;h=cb5ea106fed3e5d2135dca7d98aede13effcf7d9;hb=refs/heads/object_store
In createCacheObject, it will first find out a cache storage, in case there are multiple cache
storages available in a scope:
DataStore cacheStore = this.getCacheStorage(scope);
getCacheStorage will call StorageCacheAllocator to find out a proper NFS cache storage. So
StorageCacheAllocator is the place to choose NFS cache storage based on certain criteria,
the current implementation only randomly choose one of them, we can add a new allocator algorithm,
based on capacity etc, etc.
Regarding capacity reservation, there is already a table, called op_host_capacity which has
entry for NFS secondary storage, we can reuse this entry to store capacity information about
NFS cache storages(such as, total size, available/used capacity etc). So when every call createCacheObject,
we can call StorageCacheAllocator to find out a proper NFS storage based on first fit criteria,
then increase used capacity in op_host_capacity table. If the create cache object failed,
return the capacity to op_host_capacity.

Regarding the aging out policy, we can start a background thread on mgt server, which will
scan all the objects created on NFS cache storage(the tables called: snapshot_store_ref, template_store_ref,
volume_store_ref), each entry of these tables has a column called: updated, every time, when
the object's state is changed, the "updated" column will be got updated also. When the object's
state is changed? Every time, when the object is used in some contexts(such as copy the snapshot
on NFS cache storage into somewhere), the object's state will be changed  accordingly, such
as "Copying", means the object is being copied to some place, which is exactly the information
we need to implement LRU algorithm.

How do you guys think about the fix? If you have better solution, please let me know.



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