Since the dawn of the digital computer age, long-term data storage and backup have been the province of a single primary medium: magnetic tape. Tape has compelling advantages. It’s inexpensive to operate and buy, and even cheaper to store, whether it exists on reels, inside cartridges or as part of an automated tape library system. Tape also has the benefit of separating the portable and inexpensive storage medium from the larger, more costly recording machinery. The introduction of tape made it possible to back up everything, keep copies off-site and restore older or deleted files as needed.
In comparison, hard drive storage combined the machine and the medium into a single piece of hardware, gaining speed and simplifying access. But it also drove up storage costs and was for years simply not economical for backup use. Tape persisted as the storage medium of choice, even though it suffered from poor performance and the need for sequential, not random, access to stored data. It wasn’t very fast, and for the most part, operations had to be run in batch modes, often overnight.
In the early 1970s, IBM predicted the death of tape as a backup medium, and since then, others have continued to echo that sentiment. That hasn’t happened yet, and it’s not at all obvious that it will. But the amount of data being stored and processed continues to grow exponentially, and while ever-larger tape formats continue to emerge, the time needed to perform regular backups is also growing.
Finally, the economics of backup changed radically as hard drive storage became far cheaper. Not only are new hard drives cheap, capacious, physically smaller and increasingly reliable, but they operate much faster and offer online storage at off-line prices and with no waiting. A 250GB hard drive today costs less per gigabyte than the digital linear tape cartridges for a relatively recent tape library. Although tapes are still much more portable than RAID arrays, it’s now practical to replace tape with disk for primary backups to boost speed, improve reliability and eliminate delays in loading and searching for needed data.
One logical response to this technological change was for enterprise IT to shift to hard-drive-based backup systems. But this approach required a surprising amount of work to convert existing systems, policies and procedures. Enterprise backup teams are used to fine-tuning backup environments and applications by adding custom scripts and workflows to manage thousands of individual tapes both on- and off-site. Even positive change will be disruptive in this setting, so IT managers are rightly concerned about the effects of disk-based backups on their systems and scheduling. The better answer, at least for now, turns out to be a game of “Let’s Pretend.”
With virtual tape, even though we’re backing up direct to disk, we pretend we’re dealing with tape. Data is backed up to the disk subsystem by accessing it through what’s called a virtual tape library software that emulates the properties of tape. By making the disks look like tape, the virtual system lets IT use its existing tape-based scheduling procedures and practices, scripts and workflows; the only difference is that backup data is stored on a different set of devices. This is such a simplification as to be nearly simple-minded, but it allows IT to expand its capabilities with little or no effort, and it gets away from the need to handle, rotate and store near-line tapes. The net effect is that virtual tape makes both backups and restores faster, more reliable and cheaper.
The virtual tape libraries emulate industry-standard-based physical tape drives and libraries, presenting themselves as tape to all of the common backup software applications. A backup media server sends backup streams to a virtual tape library, which writes the data sequentially that is, in native tape format to disk storage. Through this bit of hocus-pocus, the virtual tape library appears to the system as another automated tape library, but the fact that data is being written to disk means backup jobs are completed significantly faster, often by a factor of 10 or more.
Virtual systems emulate tape operations even to the point of assigning bar codes to virtual tape “reels”or “cartridges” used by the backup software.
Virtual tape isn’t necessarily the entire answer to backup. It still doesn’t address the requirements of off-site storage and disaster recovery, but it can be used with a hierarchical storage management system in which data is moved to slower and increasingly less expensive storage media as it is used less. Virtual tape may also be included as part of a storage-area network, with a single virtual tape server managing less-used or archived data for many networked computers.
虛擬磁帶
自數字計算機時代曙光初現，數據的長期儲存和備份就一直是由磁帶這個單一的重要介質擔綱。磁帶具有引人注目的優勢。不管磁帶是盤裝的、盒式的、還是自動磁帶庫的一部分，它運營和購買成本低廉，儲存更便宜。磁帶還有另一個優點，即將可便攜的廉價介質與體積更大、成本更高的記錄機器分離。引入磁帶使備份一切東西、在機房以外的地方存放拷貝以及按需要恢復陳舊的或者刪除的文件成為可能。
比較而言，硬盤機存儲將機器與介質合成為單一的硬件，從而獲得速度和存取簡化的優勢。但是它也使存儲的成本上升，多年來對備份用途而言它不是一種經濟的方式。即便磁帶的性能不佳、只能順序而不是隨機存取儲存的數據，但磁帶作為存儲介質一直未變。它的速度不快，多數情況下，以批處理的方式運行，常常要通宵達旦地工作。
早在上世紀七十年代初，IBM就預測磁帶作為備份介質將不復存在，自那以后，其他的人也不斷附和這個論調。但這一直沒有發生，而且根本看不出明顯有這個趨勢。隨著要儲存和處理的數據繼續以指數形式急劇增加，同時更大的磁帶格式繼續出現，完成定期備份所需的時間也在增加。
最終，當硬盤存儲變得更便宜時，備份的經濟學發生了根本的變化。不僅新的硬盤機便宜、容量大、物理體積小、更可靠，而且它們運行的速度快得多，以離線的價格和無需等待時間提供在線存儲。今天，一臺250GB硬盤機的價格以每千兆字節計算，比新的磁帶庫使用的數字線性盒式磁帶更便宜。雖然磁帶仍比磁盤陣列更容易攜帶，但現在實際上已經可以用磁盤替代磁帶，作為主要的備份手段，來提高速度、改進可靠性和消除裝載和搜索所需數據時的延誤。
企業IT部門轉移到基于硬盤機的備份系統就是對此技術進步的合理響應。但是這種轉移需要做數量驚人的工作，來轉換現有的系統、政策和過程。企業備份隊伍習慣于通過增加定制的腳本和工作流來管理成千上萬現場和現場外的磁帶，精細調整備份環境和應用程序。在此轉變中，即使正面的變化也將是破壞性的，因此IT經理們要恰當地關注基于磁盤備份對系統和調度的影響。至少目前而言，較好的答案是玩“讓我們假裝”的游戲。
利用虛擬磁帶，雖然我們是直接備份到磁盤，但我們假裝是在與磁帶打交道。通過一個叫虛擬磁帶庫的軟件（它模擬磁帶的特性），將數據備份到磁盤分系統上。通過將磁盤看成像磁帶那樣，虛擬系統讓IT部門使用其已有的調度程序和做法、腳本和工作流。惟一的差別是備份數據儲存在不同的設備上。這種簡化看來好像很愚蠢，但它讓IT部門不花什么力氣就能擴展其能力，避免處理、轉動和儲存大量的磁帶。虛擬磁帶的實際效果是使備份和恢復更快、更可靠和更便宜。
虛擬磁帶庫模擬基于工業標準的物理磁帶機和磁帶庫，讓它們自己以磁帶形式面對所有常見的備份軟件。備份介質服務器將備份流送到虛擬磁帶庫，它順序地寫數據，即以原來的磁帶格式寫到磁盤存儲器。通過這種有點欺騙的方式，對系統而言虛擬磁帶庫就是另一個自動的磁帶庫，但數據寫到磁盤上這個事實意味著備份工作顯著加快了，常常是提高了10倍甚至更高。
虛擬系統模擬磁帶的工作甚至到了這樣的地步，將條形碼分配給虛擬磁帶“盤卷”或“盒式磁帶”，供備份軟件使用。
虛擬磁帶并不一定能解決備份的全部問題。它仍然沒有滿足場外存儲和災難恢復的要求，但它能與層次式存儲管理系統一起工作，在這種管理系統中，用得不太勤的數據轉移到速度更慢、越來越便宜的存儲介質上。虛擬磁帶還可以作為存儲區域網（SAN）的一部分，用單一的虛擬磁帶服務器為多臺聯網的計算機管理用得不太頻繁的或者已歸檔的數據。
文章來源于領測軟件測試網 http://www.kjueaiud.com/
