Posts Tagged ‘hash’

ESX root Password Complexity Workaround

August 30, 2013

ESX server enforces complexity requirements on passwords and if the one you want to set up doesn’t meet them, password change will fail with something like that:

Weak password: not enough different characters or classes for this length. Try again.

You can obviously play with PAM settings to lower the requirements, but here the the tip on how to really quickly workaround that.

Simply generate a hash for you password using the following command:

# openssl passwd -1

And then replace the root password hash in /etc/shadow with the new one.

From my experience on ESX 4.1, you won’t even need to reconnect the host to the vCenter. It will continue working just fine.

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Unexpected Deduplication Impact on VMware I/O Latency

May 28, 2013

NetApp deduplication is a postponed process. During normal operation Data ONTAP only calculates hashes for the data blocks. Actual deduplication is carried out off-hours as per configured schedule. Hash calculation doesn’t affect performance in most cases. I talked about that in my previous post. NetApp states in its documentation that deduplication is a low-priority process:

When one deduplication process is running, there is 0% to 15% performance degradation on other applications.

Once I faced a situation when deduplication was configured to be carried out during business hours on one of the volumes. No one noticed that at some point volume run out of space and Data ONTAP wasn’t able to perform deduplication from that time. Situation became worse when Data ONTAP was upgraded from version 7.3.2 to 8.1.0. Now during deduplication filer tried to upgrade the fingerprint metadata to a new version at 15:00 every day with the message: “Fingerprint is being upgraded” and failed. It seems that the metadata upgrade is a very resource-intensive process and heavily affects I/O latency.

This volume was not a VMware datastore, but it sit on the same aggregate together with the several VMFS LUNs. Here what happened to the VMware I/O latency every day at 15:00 (click to enlarge):

dedup_issue_ed

I deleted the host name and the datastores names from the graph. You can see the large latency spike, which won’t turn yourVMs into kernel panic, but it’s not the thing you would want your production environment to experience every day.

The solution was simple. After space was increased on this volume, deduplication metadata upgrade performed successfully and problem went away. Additionally, deduplication was shifted to off-hours.

The simple lesson to learn: don’t schedule deduplication during the day, you never know what could possibly go wrong.

NetApp Deduplication in a Nutshell

May 12, 2013

NetApp-Dedupe2NetApp uses Write Anywhere File Layout (WAFL) filesystem which is a key for NetApp’s efficient snapshot technology. If you’re already familiar with how snapshots are implemented in Data ONTAP, then understanding underlying mechanisms of deduplication is simple. Filer calculates hash for each data block it receives and preserves this in the form of metadata on the volume level. Then according to deduplication schedule (usually on weekends), filer runs metadata processing and for each duplicate hash changes data pointer to the original block of data.

Since for each data block filer needs to calculate hash on the fly, it has its penalty. On systems with CPU loaded by less than 50% performance impact is negligible. For heavily loaded systems, where CPU is nearly 100% busy, performance impact can be around 15%. For high-end 6000 systems penalty can jump up to 35% for random writes. Heavy sequential read operations can also suffer from deduplication, because read operations can be rerouted in random way across physical storage, depending on where the original data block actually is. In general, deduplication has low impact on system performance. But you can’t use it blindly and should keep in mind that in particular cases it can slow down your storage system.

Deduplication configuration is pretty simple. First of all, you need to activate deduplication on particular volume:

> sis on “targetvol”

If you already have data on the volume, you need to scan it. Otherwise, it won’t be deduped. It’s a common mistake. Deduplication is a low priority task, but keep in mind, however, that it can slightly impact your storage performance when done during business hours, especially if you run deduplication for several volumes simultaneously.

> sis start -s “targetvol”

To show the status of deduplication for particular volume:

> sis status “targetvol”

To see the deduplication schedule:

> sis config “targetvol”

And the most pleasant command to find out how much data you’ve saved:

> df -s “targetvol”

If you want to undo deduplication, first switch it off and then undo it using the following commands:

> sis off “targetvol”
> priv set advanced
*> sis undo “targetvol”
*> priv set

I, personally, was able to achieve 40% deduplication rate for VMware VMFS datastores, which is rather impressive, considering these were mixed OS + application data LUNs.

As a final note, I would like to point out, that deduplication is suitable only for environments with high percentage of similar data. VMware is a good example of it. You won’t get any significant deduplication ratio for swap file volumes, Exchnage mailboxes or Symantec Enterprise Vault which is already deduped.

Further reading:

TR-3958: NetApp Data Compression and Deduplication Deployment and Implementation Guide: Data ONTAP 8.1 Operating in 7-Mode