Most of my posts in this series have been about features that are available in a released version of Amanda. This time, I want to share a project I’m working on right now - one that will be available in Amanda-3.2. I’m reworking the way Amanda writes its data to tape (or any other kind of storage) to make it more efficient, more reliable, and simpler to configure.
Historically, Amanda’s conservative approach to finicky tape hardware has meant that it wasted some space at the end of each tape. With the changes I’m working on, Amanda will no longer waste this space, and can also avoid some needless copying of data in most cases, with a minimum of additional risk.
Amanda’s Storage Format
Before examining the new functionality, let’s look at Amanda’s storage format. Amanda treats all storage devices like tapes1 - a set of sequentially numbered data files of arbitrary size, each composed of a sequence of fixed-size blocks. Each file begins with a one-block header that identifies the dump and gives information about its contents. The header is followed by blocks of raw data.
Amanda supports writing a dump across multiple tapes - spanning.2 The technique is this: a dump is split into a sequence of parts, and each part is written a a single file on a volume. During recovery, Amanda reads the parts in sequence, and concatenates their data to reproduce the original dumpfile. Usually all parts are the same size, and this size is generally 1-5% of the tape capacity.
Better Safe than Sorry - At a Cost
Amanda was originally designed around tape drives - in fact, if you look at the history of Linux kernel support for tape drives, it is closely intertwined with Amanda development. Tape drives are finicky beasts, and in many cases cannot distinguish the end of the tape (called EOM) from any other fatal error. Worse, tape drives employ large caches to ensure they can write continuously, and when an error occurs all of the data in that cache is lost, and there is no way for Amanda to determine how much actually made it onto the tape. Beginning a new on-tape file (writing a filemark) flushes the cache and signals any errors immediately.
Since time immemorial, then, Amanda has treated any error from the tape drive as EOM, and assumed that all data written since the last filemark is potentially corrupt. That means that the part in progress when the error occurred is logged as PARTIAL, and Amanda will start at the beginning of that part on the next tape.
The PARTIAL part is recorded in the catalog, but will not be used for recovery, so it is effectively wasted space. A little arithmetic will tell you that, on average, each tape will waste half of the part size. This is at least excusable with real tape drives; with vtapes (disk), this wasted space is completely unnecessary. Worse, vtapes are most flexible when they are kept small and dumps are spanned over many vtapes per night; but the wasted space increases linearly with the number of vtapes used.
In order to rewind a part and write it again on a new tape, Amanda also needs to keep its data somewhere, called the part cache. When the dump is on the holding disk, the holding disk acts as a part cache. Otherwise, Amanda can cache parts in memory or on disk. Caching in memory is faster, but requires a lot of RAM for a reasonable part size. Caching on disk allows larger parts, but is considerably slower.
More recent tape drives (those made in the last decade or so) have a feature called “early warning”. With this feature enabled, the drive alerts the host system when it is “near” EOM, and flushes the cache to tape. Exactly what “near” means is not specified in the SCSI standard, but in general there’s room to flush the cache and write a filemark, at least. This is sometimes called a logical EOM - LEOM.
Amanda can take advantage of this functionality to cleanly finish a part before running headlong into a physical EOM. This eliminates the wasted space for a PARTIAL part, and also eliminates the need to cache parts, since rewinding is not required. In one small change (well, OK, it’s about 4,300 lines), Amanda gets faster and uses storage space more efficiently. What’s not to love?
Better yet, all of the non-tape devices (vtapes, S3 devices, DVD-ROMs, etc.) can easily emulate LEOM, so backups to these devices will automatically benefit from this improvement.
In the Code
Three important patches toward this functionality were just committed. What remains is to set up real LEOM support for the VFS device (vtapes) and for the tape device.
VFS device can, of course, trivially emulate LEOM when it is enforcing
MAX_VOLUME_USAGE property - the vtape length. However, predicting
when a filesystem will run out of space is much more difficult. We are
still discussing options, and I would love to hear suggestions here or
on the mailing list.
As for the tape device, it will assume that LEOM is not supported unless the user configures it explicitly (with the “LEOM” device property) or we can determine support for LEOM from the operating system at runtime. Unfortunately, this is one of those areas so technical that only a half-dozen people know how it works, so it may take me some time to track down this information for non-Linux operating systems. Again, advice and assistance is welcome!
- 1This is an ages-old design decision, but one that artificially constrains Amanda’s flexibility, especially with vtapes.
- 2In fact, Amanda has supported spanning forsomething like 7 years now. Yet I occasionally see users in #amandacomplaining about this serious limitation and wondering when we’re going to do anything about it. Will 2003 be soon enough?