c# create barcode from string PERFORMING FILE SYSTEM MANAGEMENT TASKS in Font

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CHAPTER 4 PERFORMING FILE SYSTEM MANAGEMENT TASKS
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a shell script and configure with cron so that it runs automatically every night. The procedure described next shows you how to create a snapshot of an existing LVM volume. 1. In the first step, you are using the lvcreate command to make a snapshot volume for the original volume /dev/system/data. The snapshot gets the name databackup. Because the original volume is in the system volume group, the snapshot will be created from that group as well. Do this by using the command lvcreate -L500M -s -n databackup /dev/system/data. Here, the option -L500M makes the snapshot 500 MB, -s makes it a snapshot volume, and -n uses the name databackup. Finally, /dev/system/data refers to the original volume that will be captured in the snapshot.
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Problems creating the snapshot volume Make sure that the kernel module Tip
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dm_snapshot is loaded! Check this with the lsmod command, and, if it isn t loaded, load it manually with modprobe dm_snapshot.
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2. If next you want to create a backup of the volume, first mount it. Do this the same way that you would mount any other volume, such as with mount /dev/system/databackup /somewhere. 3. To create a backup from the snapshot volume, use your regular backup, or for example tar. To write a backup to a rewindable tape device, you would use tar -cvf /dev/rmt0 /somewhere. 4. Finished making the backup Then you can remove the snapshot with lvremove /dev/system/databackup. Of course, this works only after you have unmounted the snapshot device.
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Doing Magic on Your File Systems with dd
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In your life as a system administrator, it is often necessary to copy data around. If it s ordinary data, an ordinary command like cp works well enough. But if the data is not ordinary, cp just isn t powerful enough. We ll now discuss some of the options that the dd command has to offer.
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Caution If the file system metadata contains important information, like a UUID which makes it possible for you to uniquely identify a file system, dd can cause some unpredicted results. If you are using dd to copy the complete file system, you will copy information like the UUID as well, and thus you ll be unable to differentiate the old file system from the new one.
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CHAPTER 4 PERFORMING FILE SYSTEM MANAGEMENT TASKS
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Speaking in generic terms, the nice thing about the dd command is that it doesn t just copy files; it can copy blocks as well. As a simple example, I ll show you how to clone your complete hard drive. Assuming that /dev/sda is the drive that you want to clone and /dev/sdb is an empty drive that can be used as the target, the dd command is rather easy: dd if=/dev/sda of=/dev/sdb. In this example, dd is used with two parameters only: if is used to specify an input file, and of is used to specify the output file (both of which are device files in this case). Next, wait until the command is finished, and you will end up with an exact copy of the original hard drive. In this example, the contents of one device were copied to another device. A slight variation is the way that dd is used to clone a DVD or CD-ROM and write it to an ISO file. To do that, in case your optical drive can be accessed via /dev/cdrom, you can clone the optical disk using dd if=/dev/cdrom of=/tmp/cdrom.iso. And of course you can mount that ISO file as well using mount -o loop /tmp/cdrom.iso /mnt. Next, you can access the files in the ISO file from the directory where the ISO is mounted. So far we have used dd only to do things that can be done with other utilities as well. It becomes really interesting if we go beyond that. What do you think of the following example, where a backup of the master boot record (MBR) is created Just make sure that the first 512 bytes of your hard drive, which contains the MBR, is copied to some file, as in dd if=/dev/sda of=/boot/mbr_backup bs=512 count=1. In this example, two new parameters are used. First, the parameter bs=512 specifies that the block should be 512 bytes. Next, the parameter count=1 indicates that only one such block has to be copied. Without this parameter, you would copy your entire hard drive, which I don t recommend. The backup copy of your MBR may be useful if some day you can t boot your server anymore because of a problem in the MBR. If this happens, just boot from a rescue disk and use the command dd if=/boot/mbr_backup of=/dev/sda bs=446 count=1. As you notice, in this restore command, only 446 bytes are written back. This is because you may have changed the partition table since you created the backup. By writing back only the first 446 bytes of your backup file, you don t overwrite the original partition table which is between bytes 447 and 511. Now I ll show you how to extend your swap space by adding a swap file. This is useful if, in the middle of the night, you get an alert that your server is almost running completely out of memory because of a memory leak you hadn t discovered so far. All you have to do is to create an empty file and specify that it should be added to the swap space. Creating this empty file is an excellent task for the dd command. In the following command you are using dd to create a file that is filled with zeros completely by using the /dev/zero device: dd if=/dev/zero of=/swapfile bs=1024 count=1000000. This would write a file of 1 GB that can be added to the swap space using mkswap /swapfile and swapon /swapfile. In the next example of the marvelous things you can do with dd, let s use it to recover the superblock on an Ext2 or Ext3 file system. To access a file system, you need the superblock, which is a 1 KB block that contains all metadata about the file system. It normally is the second 1 KB block on an Ext3 file system. In Listing 4-12, you can see a part of the contents of the superblock as displayed with the debugfs utility.
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