Category: IO

How to Copy a Disk Over SSH?

First, let’s see why do we need to copy a disk over SSH. One is of course for backup. For example if you have a VM on a LVM partition. You want to keep a copy of the block level backups, you prefer to create a snapshot of the lvm partition and then copy the disk as an image to your backup server. The other being quite the same, but for different purpose. What if you want to migrate a VM that you have created on an LVM partition, and then you want to migrate it as a raw file to another server? Or a LVM partition to another server? For those cases, the technique is pretty awesome.

Copy The Disk to a RAW Image

First, let’s learn how can copy a disk as raw image.

For example, you are sitting in your backup server. And you want to copy a disk or lvm partition or a partition of a disk, from a remote server to your backup server. And you want to keep the copy of the image, then you may run dd command as following from your backup server:

ssh [email protected] "dd if=/dev/vg0/v1092-kdkdksjuekksq" | dd of=/backup/v1092.raw

In our case, 10.10.10.10 is the IP of the server, that partition/disk currently reside. We are trying to copy a LVM partition namely: /dev/vg0/v1092-kdkdksjuekksq, just replace this one with the one your desired lvm partition. You may also do this for a disk like /dev/sda, the command would be like the following then:

ssh [email protected] "dd if=/dev/sda" | dd of=/backup/v1092.raw

Now as you have copied the disk/partition, you may look at what the partition holds by checking the data inside it. To know, how to mount a raw disk image, you may check the following:

How-to-Mount-raw-VM-disk-images-KVMorXenorVMW

Copy The Disk To Another Remote Disk over SSH

You may either copy the disk directly to a secondary disk you have on the backup/migrated server or dump the image that you copied to another disk/partition of same size or bigger. To copy directly a partition /dev/sda from another server, to a backup server with the secondary disk /dev/sdb, you may do the following:

ssh [email protected] "dd if=/dev/sda" | dd of=/dev/sdb

This would backup /dev/sda from 10.10.10.10 and restore to /dev/sdb drive you have on the server that ran the command.

To just dump the image file, that copied earlier on the other example to /dev/sdb, you may do the following:

dd if=/backup/v1092.raw of=/dev/sdb

Hope this helps.

How to: Setup a server for R1Soft CDP backup?

We at Mellowhost has been utilizing R1Soft CDP backup for last 8 years. R1Soft has been a great backup tool even though the tool is immensely resource hoggy. At different times we had gone through different situations to handle our backup servers efficiently. After all the hiccups with backup nodes, we ended up efficiently configuring 3 backup servers of 3 different configuration

  1. backup1 = It contains 12TB file system on a RAID 0 array. It copies data to a BTRFS compressed drive once a week to keep the data safe if RAID 0 dies. This server uses RAID 0 for faster drive verification and block scanning by r1soft. This server hosts servers that requires frequent backing up and can sustain a loss of a week data (Less important data). As the server performs really fast due to being RAID 0, we can run multiple r1soft threads at a time including disk safe verification and block scans.
  2. backup2 = It contains 30TB file system in RAID 6 hardware array. This is used for hosting our VPS backups. This server is a seriously large one to keep backups of our enterprise VPS clients.
  3. backup3 = It contains 16TB file system in RAID 10 hardware array. This server is hosted in a East Coast American Location. It is our off network backup server and keeps backups for East Coast servers too.

One of the key factor in designing a backup server is the size and the location. Need to keep in mind that CDP 3 takes more space than CDP 2 for unknown reason while still being a differential backup solution, not just an incremental. Location of the server matters due to the network speed. If you are hosting your server a lot far than the server network, it may take longer time to complete the initial storage. Due to the latency it may fails to perform as fast like 1Gbps even if both network supports it. Just for an example, if you are backing up your data at 1MBps speed, it would take 12.13 days to complete backup of 1TB data [ Calculation: (((1024 x 1024) / 60) / 60) / 24 = 12.13 days ]. A 100Mbps port can give you speed upto 10MBps, while you can have 50MBps+ speed if you are using a 1Gbps network roughly. So why does the speed matter? If you are backing up your initial data in 13 days, that doesn’t mean it will be the same all the time. Your second backup would take much less amount of time as it only needs to upload the differential backups. That is true! But the problem will come when you require to do a bare metal restore. If your server requires a disaster recovery, you would then need 13 days to restore your server to the original state. Your customers won’t sit down for 13 days! While creating backup, it is important to think about disaster recovery too. How fast are you going to be able to restore the backup is an important concern while designing your disaster recovery solution.

I always recommend users to choose a 1Gbps network with a latency below 2ms if you want to have a good disaster recovery solution. This can guarantee a faster bare metal restore when needed.

The second key factor while creating the R1soft backup server would be to choose the RAID. If you are thinking to create r1soft backup on a non-raided solution, I think you should drop off your idea. RAID isn’t necessarily always use to keep your data safe, it can also be used for performance. Keeping a RAID 0 or striping in general is must for a R1Soft server. Otherwise, every couple of times, you are going to see a lot of stalled processes doing ‘disk safe verification’ ‘block scan’ etc etc and not able to keep the backup up to date or canceling processes due to duplicate backup process (Old one taking too long to complete). It is better not to choose RAID 5. I particularly didn’t try RAID 5, but I have used RAID – Z on ZFS file system, which was seriously slow for my work around. I switched the server later on to RAID 0 and BTRFS compression to keep a weekly backup which tremendously improved the R1Soft performance. We at later time, worked to create more backup servers with hardware RAID WB cache and battery backed unit to give us more performance benefit while creating and restoring backups. These servers have been performing tremendously well with R1Soft. They can also be called good disaster recovery node.

Last, I recommend you to understand that backup isn’t just keeping a copy of your data of your online existence. It is important to design a disaster recovery solution instead of just creating backups. If you are simply into creating backups, you probably don’t need R1Soft or any high end servers instead simple Rsync would work fine. But to create ‘Disaster Recovery’ solution, you need high level planning, good hardwares and good cost estimation. If you are leaving behind in any, you will probably fail to create a good disaster recovery solution that actually ‘works’.

How to: Find dm number of a LVM logical volume

Sometimes, you will see the error thrown in dmesg or /var/log/messages are mentioned in dm-number format, while you manage the disk using lvm logical volume name. This is because lvm logical volumes are designed through kernel device mapper technique and kernel recognizes volumes using dm numbers. There is a tool to list all the device mappers used for block devices under Linux. Simply type the following to list the maps:

# lsblk

It shall show something like the following:

There you can see the dm number for each lvm volume is listed under first bracket. For example the swap in this server is created with LVM with the name vg_iof442/swap and has the dm-1 mapping.

How to: Find IOPS usage in a Linux Server

Question: How to find iops usage of a linux server?

Answer: Use iostat. Iostat is a tool comes with the ‘sysstat’ package. If you type iostat on your CentOS/Redhat server and it says the command not found, you can install sysstat to avail the iostat command.

yum install -y sysstat

An example iostat usage case could as simple as following:

iostat -x 1

-x tells iostat to give extended statistics which is required to find read/write iops individually. And the 1 tells iostat to repeat the command every 1s.

An example output would be like the following:

If you look at the output, the colum r/s would say the read iops and the colum w/s would say write iops. If you are using simple ‘iostat 1’ then the column tps should show the total iops of the disk in use.

If you are using a spinning disk, and if you are getting anything around 150-200 cumulatively, you are probably hitting the iops limit. With raid, the number would change according to your raid choice. Although, the number can increase in case of using Writeback SSD Cache, Hardware RAID Cache or Pure SSD disks. Most important benefit of using SSD is not essentially the amount of throughput it gives in a practical environment instead the amount of IOPS it can sustain is phenomenal.