{"id":4774,"date":"2013-04-22T16:58:43","date_gmt":"2013-04-22T06:58:43","guid":{"rendered":"http:\/\/nsrd.info\/blog\/?p=4774"},"modified":"2018-12-11T14:12:46","modified_gmt":"2018-12-11T04:12:46","slug":"basics-tape-capacity-and-compression","status":"publish","type":"post","link":"https:\/\/nsrd.info\/blog\/2013\/04\/22\/basics-tape-capacity-and-compression\/","title":{"rendered":"Basics – Tape capacity and compression"},"content":{"rendered":"

Tape capacity.<\/p>\n

It should be straight forward, but many people tend to look at only the biggest, most optimistic numbers, and assume the best. In backup, this is a mistake, and one which I’ve seen consistently made the entire time I’ve been working in the industry.<\/p>\n

These days, when it comes to tape, LTO is the king of the hill, so consider the following table outlining native (i.e., uncompressed) format capacities:<\/p>\n

\"LTO<\/a>
LTO Capacity, Uncompressed<\/figcaption><\/figure>\n

This gives a baseline<\/em> figure of tape capacity. But, LTO is capable of in-drive compression, which means vendors don’t quote<\/em> uncompressed capacity, they quote compressed<\/em> <\/em>capacity (and for that matter, speed).<\/p>\n

So the tables we see look more like the following:<\/p>\n

\"LTO<\/a>
LTO Capacity, compressed<\/figcaption><\/figure>\n

See the much bigger capacity sizes? See the tiny footnote? That’s called blind optimism<\/i>. In actual fact, it’s actually getting worse, not better, since LTO-6 saw a change in the compression algorithm used. That means vendors are now quoting a compressed capacity of 2.5:1 \u2013 so that LTO-6 there should in fact read 6,250 GB, not 5000 GB.<\/p>\n

Unfortunately, a lot of people don’t look at the footnotes \u2013 or if they do, they plan, with wild optimism, to hit that 2:1 compression ratio every time. And when they don’t it’s met with consternation and surprise.<\/p>\n

The truth is, compression ratios are a bit of a joke on tape, since they’re based entirely<\/em> on what sort of data you’re backing up. I have, on multiple occasions, seen 1TB written to LTO-1 and LTO-2 tapes. Not once \u2013 not once<\/em> have I worked under impression that I should get that compression ratio all the time. In case you’re wondering, these came from very large databases (at the time) that had been preallocated \u2013 so we were literally backing up very large, very empty, and thus very compressible files.<\/p>\n

Whenever I’m sizing for a customer, I use one compression ratio, and one compression ratio only, regardless of the tape being used, and that’s 1.3:1. Sometimes it’s pessimistic, and sometimes it’s optimistic, but on the whole, it’s usually fairly accurate. So on an LTO-5 tape, I’m assuming that in all likelihood, we should see a compressed used capacity of around 1,950GB. Anything after that? Cream on top. Anything below that? Heavy data.<\/p>\n

Don’t let yourself be fooled by tape compression ratios: they’re there for marketing purposes, not to be reflective of reality.<\/p>\n","protected":false},"excerpt":{"rendered":"

Tape capacity. It should be straight forward, but many people tend to look at only the biggest, most optimistic numbers,…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[6],"tags":[242,980],"class_list":["post-4774","post","type-post","status-publish","format-standard","hentry","category-basics","tag-compression","tag-tape"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t