{"id":1086,"date":"2008-02-05T17:39:45","date_gmt":"2008-02-05T17:39:45","guid":{"rendered":"\/blogs\/paul\/post\/Search-Engine-QA-15-Mirrored-backups.aspx"},"modified":"2008-02-05T17:39:45","modified_gmt":"2008-02-05T17:39:45","slug":"search-engine-qa-15-mirrored-backups","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/search-engine-qa-15-mirrored-backups\/","title":{"rendered":"Search Engine Q&A #15: Mirrored backups"},"content":{"rendered":"


\n

This post is based on one from my old MSDN blog but the topic has come up a few times in recent days so I want to revamp it and re-post.<\/P>
\n

There are two things that confuse people about mirrored backups – can you mix-n-match backup devices from the mirrors, and what exactly do the various sizes mean?<\/P>
\n

1) Single-device backup, no mirror<\/STRONG><\/P>
\n

The code below creates a single-device backup with no mirror, and then examines it.<\/P>
\n


\n

BACKUP DATABASE<\/FONT> AdventureWorks <\/FONT>TO
DISK <\/FONT>=<\/FONT> N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’
<\/FONT>WITH<\/FONT> FORMAT<\/FONT>, <\/FONT>STATS<\/FONT>;
<\/FONT>GO<\/FONT><\/P>RESTORE HEADERONLY FROM DISK <\/FONT>= <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’<\/FONT>;
<\/FONT>GO<\/FONT>
\n

<\/P><\/BLOCKQUOTE><\/FONT>
\n

The BackupSize<\/EM> in the HEADERONLY<\/FONT> output is 168,899,072 bytes and the on-disk size of the file mediaset1device1.bck<\/FONT> is 161MB.<\/P>
\n

2) Single-device backup, mirrored<\/STRONG><\/P>
\n

The code below creates a single-device backup with a mirror, and then examines it.<\/P>
\n


\n

BACKUP DATABASE<\/FONT> AdventureWorks <\/FONT>TO
DISK <\/FONT>=<\/FONT> N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’
MIRROR<\/FONT> TO DISK<\/FONT> = <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset2device1.bck’
<\/FONT><\/FONT>WITH<\/FONT> FORMAT<\/FONT>, <\/FONT>STATS<\/FONT>;
<\/FONT>GO<\/FONT><\/P>RESTORE HEADERONLY FROM DISK <\/FONT>= <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’<\/FONT>;
RESTORE HEADERONLY FROM DISK <\/FONT>= <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset2device1.bck’<\/FONT>;<\/FONT>
<\/FONT>GO<\/FONT><\/FONT><\/FONT><\/FONT><\/BLOCKQUOTE><\/FONT><\/FONT>
\n

The BackupSize<\/EM> in the HEADERONLY<\/FONT> output of both files is 337,798,144 bytes. This is double the size of the backup in case #1 above – and it because there are now two copies of the backup. The on-disk size of both files is 161MB, which is what we’d expect as mediaset2device1.bck is a copy of <\/FONT>mediaset1device1.bck.<\/FONT><\/FONT><\/FONT><\/FONT><\/P>
\n

3) Two-device backup, no mirror<\/STRONG><\/P>
\n

The code below creates a two-device backup with no mirror, and then examines it.<\/P>
\n


\n

BACKUP DATABASE<\/FONT> AdventureWorks <\/FONT>TO
DISK <\/FONT>=<\/FONT> N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’,<\/FONT>
<\/FONT><\/FONT>DISK <\/FONT>=<\/FONT> N<\/FONT>‘C:\\SQLskills\\mediaset1device2.bck’<\/FONT>
<\/FONT>WITH<\/FONT> FORMAT<\/FONT>, <\/FONT>STATS<\/FONT>;
<\/FONT>GO<\/FONT><\/P>
\n

RESTORE HEADERONLY FROM DISK <\/FONT>= <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’<\/FONT>;
<\/FONT>GO<\/FONT><\/FONT><\/P><\/FONT><\/FONT><\/FONT><\/BLOCKQUOTE>
\n

The BackupSize<\/EM> in the HEADERONLY<\/FONT> output is 169,959,424 bytes. This is nearly exactly the same as for the single-device backup in case #1, but includes a bit more to account for the extra metadata in the second device. This time, the on-disk size of the file mediaset1device1.bck<\/FONT> <\/FONT>is 81MB. This is half of the on-disk size from the single-device case #1 as the backup is now split between the two files.<\/P>
\n

4) Two-device backup, mirrored<\/STRONG><\/P>
\n

The code below creates a single-device backup with a mirror, and then examines it.<\/FONT><\/P>
\n


\n

BACKUP DATABASE<\/FONT> AdventureWorks <\/FONT>TO
DISK <\/FONT>=<\/FONT> N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’,<\/FONT>
DISK <\/FONT>=<\/FONT> N<\/FONT>‘C:\\SQLskills\\mediaset1device2.bck’
<\/FONT>MIRROR<\/FONT> TO DISK<\/FONT> = <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset2device1.bck’,
<\/FONT>DISK<\/FONT> = <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset2device2.bck’<\/FONT>
<\/FONT><\/FONT>WITH<\/FONT> FORMAT<\/FONT>, <\/FONT>STATS<\/FONT>;
<\/FONT>GO<\/FONT><\/P>
\n

RESTORE HEADERONLY FROM DISK <\/FONT>= <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’<\/FONT>;
RESTORE HEADERONLY FROM DISK <\/FONT>= <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset2device1.bck’<\/FONT>;<\/FONT>
<\/FONT>GO<\/FONT><\/FONT><\/FONT><\/FONT><\/FONT><\/P><\/BLOCKQUOTE>
\n

<\/FONT>The BackupSize<\/EM> in the HEADERONLY<\/FONT> output of both files is 339,918,848 bytes – again, double the size of the non-mirrored backup in case #3. The on-disk size of each file is 81MB, as each file is one half of a copy of the backup.<\/FONT><\/P>
\n

Restoring<\/FONT><\/STRONG><\/P>
\n

<\/FONT>Now let’s try to mix devices from the two backup media sets and see if it’s possible:<\/P>
\n


\n

RESTORE<\/FONT><\/FONT> <\/FONT>DATABASE<\/FONT><\/FONT> AdventureWorks
<\/FONT><\/FONT>FROM<\/FONT> <\/FONT>DISK <\/FONT>= <\/FONT>N<\/FONT>‘C:\\SQLskills\\mediaset1device1.bck’<\/FONT><\/FONT>,
<\/FONT>DISK <\/FONT>= <\/FONT>N<\/FONT><\/FONT>‘C:\\SQLskills\\mediaset2device2.bck’
<\/FONT>WITH<\/FONT> <\/FONT>REPLACE<\/FONT>,<\/FONT> STATS<\/FONT><\/FONT>;
<\/FONT>GO<\/FONT><\/P><\/BLOCKQUOTE><\/FONT><\/FONT>
\n

And it works fine – excellent! That’s the whole point of having mirrored backups.<\/P>
\n

One other question is – can backup device types can differ between media sets in the same backup. The answer to this is no – as documented in Books Online. All the backup devices involved in a single backup, regardless of whether they’re part of a mirror media set or not, must be of the same type and have similar characteristics.<\/P>
\n

Hope this is useful.<\/P><\/FONT><\/p>\n","protected":false},"excerpt":{"rendered":"

This post is based on one from my old MSDN blog but the topic has come up a few times in recent days so I want to revamp it and re-post. There are two things that confuse people about mirrored backups – can you mix-n-match backup devices from the mirrors, and what exactly do the […]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[13,38,78],"tags":[],"class_list":["post-1086","post","type-post","status-publish","format-standard","hentry","category-backuprestore","category-example-scripts","category-search-engine-q-and-a"],"yoast_head":"\nSearch Engine Q&A #15: Mirrored backups - Paul S. Randal<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.sqlskills.com\/blogs\/paul\/search-engine-qa-15-mirrored-backups\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Search Engine Q&A #15: Mirrored backups - Paul S. Randal\" \/>\n<meta property=\"og:description\" content=\"This post is based on one from my old MSDN blog but the topic has come up a few times in recent days so I want to revamp it and re-post. There are two things that confuse people about mirrored backups – can you mix-n-match backup devices from the mirrors, and what exactly do the […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.sqlskills.com\/blogs\/paul\/search-engine-qa-15-mirrored-backups\/\" \/>\n<meta property=\"og:site_name\" content=\"Paul S. 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