Original link:
http://blog.chinaunix.net/uid-15007890-id-106818.html
Snapshot is a point-in-time data copy technology and an important concept in the field of data backup . Understanding snapshots and managing them effectively is an important lesson in disk management.
This article will describe how to apply and manage snapshots in the disk management tool Storage Foundation.
The purpose of the snapshot is to record the data information at a certain time and save it. If some failure occurs and data recovery is required, the snapshot can be used to restore the data to the state of the previous time point , and the data after the time point can be restored. Data will be lost. Backup system is one of the main application areas of snapshot technology . When backup software needs to back up some critical business that cannot be stopped, snapshot technology can be used to save and back up all data information at a certain point in time without affecting the business. normal operation.
Snapshot technology is divided into two categories: physical copy and logical copy, physical copy is a complete copy of the original data ; logical copy is to copy only the data that has changed . Although both copy technologies can restore data to a certain point in time, each has its own disadvantages:
The advantage of physical copy is that it is simple to manage, and it does not need to monitor the status of the target data, directly copy all the data to another place, and can directly save it as a data backup. Its disadvantage is that it requires the largest storage space, and requires as much space as the target data to copy it completely.
The advantage of logical copy is to save space. Generally speaking, frequently changed data only accounts for 20%-30% of all data, so logical backup can save about 70% of storage space. But logical backup also has its shortcomings, because it only saves the changed data, so if the target data is damaged, the snapshot can't do anything. The copy-on-write technology that is popular in current file systems and backup software is a logical copy.
Although snapshot technology has been widely used in the storage industry, many users will misunderstand it. Now let's explain some common problems:
Snapshot vs. Mirror
Physical copy snapshots and mirroring work in the same way. They both completely copy the contents of a target data source to another place. However, snapshots are copied at a certain point in time, and changes in the target data will no longer be affected. record, and mirroring is to ensure the consistency of target data and copy data at all times.
The purpose of the two is also different. The purpose of snapshot is to be able to restore to the previous one when the system fails, while the purpose of mirroring is to ensure data redundancy and quickly recover when the data source fails. If the user deletes a file by mistake, if the user has taken a snapshot before, it can be recovered; if the user has made an image, the file under the image file will also be lost and cannot be recovered.
Conversely, if the user's target data source is damaged and all data is lost, the snapshot can only be restored to the most recent snapshot, and the newly modified data will be lost, while the mirror can quickly restore all data to ensure business continuity. .
Logical copy snapshots and mirroring are completely different and cannot be compared.