Bryce Wade 讲了一个与备份索引有关的恐怖故事，Bryce 这个故事当中的场景是要备份一台大的 NFS 服务器，这台服务器的角色是存放 Web 文件，据说有 300 多万个小文件，这样造成备份索引超大，索引经常损坏，客户把索引分布到多个从服务器（slave servers）上，索引损坏还是经常发生。为保证备份索引的可用性，客户每天把索引导出成 ascii 文件，然后对这个 ascii 文件进行备份。这真是备份之备份啊
The importance of the backup index, and your familiarity with how it works, cannot be
overemphasized. It is the lifeblood of any backup system and should be treated like
gold.
Most of my backup horror stories stem from problems with the backup index. My first
bad experience was with backing up a large NFS server that was used to store home pages
for a large online service’s web servers. There were more than three million small
files, which made the index so large that it would often become corrupted. Even after
distributing the index over multiple slave servers, the size would still cause index
corruption. As if regular index corruption weren’t enough, we often would not catch the
corruption until several days later when the backup product would act stranger than
usual. Since we were foolish enough to keep only two days’ worth of index backups, we
could not recover a reliable index. Eventually, we ended up dumping the index into ASCII
files daily and then backing up those files from a different server with the regular
retention schedules.
My other index horror story comes from the same site. In another effort to keep our
index small, we stored the index of a backup for only two weeks (even though the data
was kept on a backup volume for two months). I had one user who on multiple occasions
deleted data after he was done with it, only to determine two and a half weeks later
that he really wasn’t done with it. Since the records containing those files had expired
out of the index, every volume that might have the data had to be reread. One of those
restores had me rereading more than 40 DLT 4000 tapes (in a jukebox that held only 28
tapes) while still trying to do regular backups. It took me more than three long days to
read the tapes; even then I was not able to retrieve all of the data. Fortunately for my
job, it was not mission-critical data.
Bryce Wade —— From W.Curtis Preston《Backup & Recovery》8.16. Protection of the Backup Index
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