You're absolutely right that anything over the "raw" capacity is a matter of luck, and that discounting marginal media.
However, there are a couple of rules of thumb that I find useful: - for the average client, a net compression ratio of about 1.5:1 is fairly normal. 2:1 or higher is rare. - the "compressed" rating is actually a fairly good sizing indicator of tape vs disk capacity. When you allow for not running hard drives over 80% full, and reasonable expectation of compression, the upper value is about the size of the disk space you can plan to protect with it, assuming full backups. /kenw > -----Original Message----- > From: Ben Scott [mailto:[EMAIL PROTECTED] > Sent: February-15-08 12:32 PM > To: NT System Admin Issues > Subject: Re: Backupexec and compression > > On Fri, Feb 15, 2008 at 1:25 PM, Joseph L. Casale > <[EMAIL PROTECTED]> wrote: > > Exactly? They are 200/400 tapes. > > I ignore "compressed capacity" claims; they are a marketing gimmick > and always have been. Compression rates vary tremendously. You might > get no compression gain, or 1.5:1, or 3:1. So I just avoid quoting > "compressed capacity" entirely. > > By "200/400 tape", I surmise you LTO-2. Those do have a nominal, > native capacity of 200 GB. But in the land of disks and tapes, "200 > GB" means "200 billion bytes". In the land of software and operating > systems, "200 GB" means "200 * 2^30 bytes". This is sometimes > explicitly written as "200 GiB". Compare: > > 200 GB = 200,000,000,000 bytes > 200 GiB = 214,748,364,800 bytes > 14 GiB = 14,748,364,800 bytes (difference) > > So right away, you're only going to get 93% of that "200 GB" you > thought you had. > > Then there is overhead for metadata. Is this a large number of > small files? If so, the metadata for each file may be becoming > significant. I don't know anything about BUE's on-tape format, but I > would guess they are likely storing a header at the start of each file > (with name, datestamps, size, NTFS ACL, etc.). Or maybe in a catalog > at the start/end of the tape. Either way, it consumes tape space. > > You also loose some tape capacity to bad blocks. Any big tape is > going to have some number of bad blocks. Nominally, the drive > automatically detects and corrects for them, so the backup software > (and you) are not aware it is even happening. But it can eat into the > nominal capacity of the tape. Is it an old tape? > > Finally, yes, a bad or simplistic compression algorithm may well > make the "compressed" data larger for already-compressed input. A > smart compression implementation will detect this and just store the > straight input data, but tape drives are not known for using smart > implementations. So turning off hardware compression may well gain > you some tape space back. But perhaps not as much as you think. > > -- Ben > > ~ Upgrade to Next Generation Antispam/Antivirus with Ninja! ~ > ~ <http://www.sunbelt-software.com/SunbeltMessagingNinja.cfm> ~ ~ Upgrade to Next Generation Antispam/Antivirus with Ninja! ~ ~ <http://www.sunbelt-software.com/SunbeltMessagingNinja.cfm> ~
