NetApp Reallocate

The smallest addressable block of data in Data ONTAP is 4k. However, all data is written to volumes in 256k chunks. When data block which is bigger than 256k comes in, filer searches for contiguous 256k of free space in the file system. If it’s found, data block is written into it, if not then filer splits the data block and puts it in several places. It’s called fragmentation and is familiar to everyone from the times, when FAT files ystems were in use. It’s not a big issue in modern file systems, like NTFS or WAFL, but defragmentation can help to solve performance problems in some situations.

In mostly random read/write environments (which is quite common these days) fragmentation has no impact on performance. If you write or read data from random places of the hard drive it doesn’t matter if this data is random or sequential on the physical media. NetApp recommends to consider defragmentation for the applications with sequential read type of workload:

• Online transaction processing databases that perform large table scans
• E-mail systems that use database storage with verification processes
• Host-side backup of LUNs

Reallocation process uses thresholds values to represent the file system layout optimization level, where 4 is normal and everything bigger than 10 is not optimal.

To check the level of optimization for particular volume use:

> reallocate measure –o /vol/vol_name

If you decide to run reallocate on the volume, run:

> reallocate start –f /vol/vol_name

There are certain considerations if you’re using snapshots or deduplication on volumes. There is a “-p” option, to prevent inflating snapshots during reallocate. And from version 8.1 Data ONTAP also supports reallocation of deduplicated volumes. Consult official documentation for additional information.

Further reading:

TR-3929: Reallocate Best Practices

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NetApp thin-provisioning for VMware LUNs

LUN and Volume Thin Provisioning

I already described thin provisioning of VMware NFS volumes some time ago here. Now I want to discuss thin provisioning of LUNs.

LUNs are different from VMFS on top of NFS implementation, because LUN is an additional container inside of NetApp FlexVol. So if you’re using FC, you need to thin provision both LUN and volume:

> lun set reservation “/vol/targetvol/targetlun” disable
> vol options “targetvol” guarantee none

In fact, you can make the LUN thin and the volume thick. Then storage space that’s not used by the LUN, is returned to the volume level. But in this case it cannot be used by other volumes as a shared pool of space.

As the best practice, NetApp now recommends to set Fractional Reserve and Snap Reserve for your volumes to 0%. Don’t forget about that, if you want to save more storage space:

> vol options “targetvol” fractional_reserve 0
> snap reserve “targetvol” 0

Disable snapshots if you don’t use them:

> snap sched “targetvol” 0

It’s easy as that. Now you don’t waste your space by reserving it ahead, but use it as a shared pool of resources. But make sure to monitor aggregate free space. If you starting to run out of storage, plan purchase of new disks in advance or redistribute data between other aggregates.

Safety Features

Disabling volume level, LUN and snapshot reservations helps you to save storage space. The drawback of this approach is that you don’t have any mechanisms in place to prevent volume out-of-space situations. If you enable snapshots on the volume and they consume all the volume space, the volume goes offline. Very undesirable consequence. NetApp has two features that can serve as safety net in thin-provisioned environments: autosize and snap autodelete.

Snap autodelete automatically removes old snapshots if there is no space left inside the volume. Autosize, on the other hand, allows the volume to automatically grow to the specified limit (+20% to the volume size by default) in specified increments (5% of the volume size by the default). You can also specify what to do first autosize or snapshot autodelete by using ‘try_first’ option.

> snap autodelete “targetvol” on
> vol autosize “targetvol” on
> vol options “targetvol” try_first volume_grow

SnapMirror Considerations

If you use SnapMirroring and switch on the autosize on the source volume, then the destination volume won’t grow automatically. And SnapMirror will break the relationship if it runs out of space on the smaller destination volume. The trick here is to make the destination volume as big as the autosize limit for the source volume and thin provision the destination volume. By doing that you won’t run out of space on destination even if the source volume grows to its maximum.

Further reading

TR-3965: NetApp Thin Provisioning Deployment and Implementation Guide Data ONTAP 8.1 7-Mode

NetApp thin provisioning for VMware

Thin provisioning is a popular buzzword, especially when it comes to NetApp. However, it can really save you time and headache in a number of situations. We use thin provisioning while presenting NFS volumes from NetApp to VI3 ESX hosts. Well, NetApp already let you change size of its FlexVol volumes on the fly. But you need to do it manually. Thin provisioning helps you to configure volumes so that in case of space shortage on a volume it will automatically expand without manual intervention. Of course you need to look after your volumes, otherwise they can fill all your storage space. But it will save you enough time to resolve data growth problem. Without thin provisioning in such situation your applications can easily crash.

NetApp doesn’t support iSCSI thin provisioning for VMware, so NFS is the only option. Don’t be afraid of performance issues. Without a doubt it’s slower than FC, but NetApp is famous for its NFS performance and it’s very well suited for mid-level workloads.

To be more specific, using thin provisioning you can create say 300GB virtual hard drive for particular VM and it will initially use no space. Then it will grow as long as you fill it. It can save you tremendous amount of storage space. Because you never exactly know ahead how much space you need. But be aware, if you will try to migrate thin provisioned virtual hard drive using storage migration plugin for VMware Virtual Center then it will fill all space. It means 300GB will use all 300GB even if it’s half-full.

The best article which will help you to integrate NetApp with VMware VI3 is NetApp TR-3428: NetApp and VMware Virtual Infrastructure 3 Storage Best Practices. What I will write here are basically excerpts from this article.

NetApp Configuration

Lets start from the NetApp configuration. First thing to do is to disable snapshots as usual. Generally it’s not a good idea to make snapshots of VMware virtual hard drives on the fly. They won’t be consistent. I will touch this topic in my later posts.

> snap sched <vol-name> 0 0 0
> snap reserve <vol-name> 0

Next step is to disable access time update on the volume, which is safe because VMware doesn’t rely on accurate access time for its files. It will increase performance, since Filer won’t need to update access time for files each time they are read or written.

> vol options <vol-name> no_atime_update on

Then configure the thin provisioning feature itself by switching volume auto size policy to on. It has two keys -m and -i. By -m you set maximum volume size and by -i you configure increment size.

> vol autosize <vol-name> [-m <size>[k|m|g|t]] [-i <size>[k|m|g|t]] on

NetApp recommends to disable Fractional Reserve for thin provisioned volumes, it’s just not needed anymore. Fractional Reserve guarantees successful writes to volumes in case you use snapshots. According to how snapshots work if you completely overwrite snapshot data you will use double amount of storage space. And it’s where Fractional Reserve comes into place. It reserves 100% of additional space for such cases. It means you will never run into situation when you are out of space due to active snapshots. But since we enabled auto size, our volume will resize on demand and Fractional Reserve becomes redundant. Supposedly auto size was implemented little bit later than Fractional Reserve and we have both of them in NetApp.

> vol options <vol-name> fractional_reserve 0

In case you use snapshots as a tool for instant VMware block level backups you can change auto delete policy. I said earlier that you should disable snapshot schedule, however you can manually (using scripts) create consistent snapshots. If you want to do that then you can additionally instruct NetApp to delete oldest snapshots when you are out of space on Filer and can’t auto grow volume.

> snap autodelete <vol-name> commitment try trigger volume target_free_space 5 delete_order oldest_first
> vol options <vol-name> try_first volume_grow

Now we need to create NFS export on NetApp Filer. It’s where FilerView interface comes handy. In short, you should give your ESX hosts read-write access, root access and configure Unix security style.

VMware Configuration

VMware configuration is trivial. Go to VMware Add Storage Wizard, select Network File System, then point to your NetApp filer and specify your volume path. Additionally NetApp recommends to tune NFS heartbeat parameters. Go to Host Configuration – Advanced Settings – NFS and for ESX 3.0 hosts change:

NFS.HeartbeatFrequency to 5 from 9
NFS.HeartbeatMaxFailures to 25 from 3

For ESX 3.5 hosts change:

NFS.HeartbeatFrequency to 12
NFS.HeartbeatMaxFailures to 10

There are much more information and tuning parameters that you might want to read about. Find some time to look through TR-3428 in case you need some clarifications or additional info.