TiVo Community Forums Archives - Ok, need some advice...

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Ok, need some advice...
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Posted by: gelee

Did a backup to a single compressed file using "dd if=/dev/hdc bs=1024k | gzip -9c > /mnt/tivodisk.gz". Call this backup #1. Did a second backup to compressed split files using "dd if=/dev/hdc bs=1024k | gzip -9c | split -b650m - /mnt/tivodisk_". Call this backup #2. dd reports no errors in either case. When I split backup #1 and do a CRC check on the first file, it's different from the CRC check in the first file from backup #2. The remaining files have matching CRCs. Now I don't know which backup is good. Shouldn't they both have identical CRC values?

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Posted by: gelee

Ok, now I've done a total of 4 backups, each with different results:

Backup1
3699984080 681574400 tivodisk_aa
4110521594 681574400 tivodisk_ab
1830263269 681574400 tivodisk_ac
1045016399 681574400 tivodisk_ad
3100562364 137996329 tivodisk_ae

Backup2
2484486673 681574400 tivodisk_aa
4110521594 681574400 tivodisk_ab
1830263269 681574400 tivodisk_ac
1045016399 681574400 tivodisk_ad
3100562364 137996329 tivodisk_ae

Backup3
1748050260 681574400 tivo_aa
4110521594 681574400 tivo_ab
1830263269 681574400 tivo_ac
1045016399 681574400 tivo_ad
3100562364 137996329 tivo_ae

Backup4
730365927 681574400 tivodisk_aa
4110521594 681574400 tivodisk_ab
1830263269 681574400 tivodisk_ac
1045016399 681574400 tivodisk_ad
3100562364 137996329 tivodisk_ae

CRC value in the 1st column, file size in the 2nd column.

Anyone have an explanation?

Posted by: CoffeeBreath

quote:
Originally posted by gelee:
Ok, now I've done a total of 4 backups, each with different results:

I'd guess one or more bad blocks on your Tivo disk. It would be interesting to narrow it down to a smaller range: split your first chunks into several, and see which checksums differ among the pieces.

Don't IDE drives do automatic bad-block revectoring? Haven't kept up on the technology recently... :-)

-CB

Posted by: Dr. Zed

My advice would be to purchace a second drive to use and keep your original as a backup.

If you don't want to make such a purchace, try restoring your backup to a blank drive and test it to see how it works. For example, if you have a drive reserved to be your B drive, try using it as a restore A drive for a week. If it works ok, then you know your backup is sound. However, I believe that A drives have different requirements than B drives, so your second drive may not be adaquate to act as an A drive.

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"It's like living in the future."™

Posted by: Tiger

Damn, I wish I'd read this thread earlier.. There is absolutely nothing wrong with your backup. dd would have reported errors. What you are seeing is gzip storing a timestamp. To demonstrate, I did the following:

gzip -c -9 < file > file.gz1
gzip -c -9 < file > file.gz2
gzip -c -9 < file > file.gz3

Then I did md5 file.gz* to get a checksum on the files, and got:

MD5 (file.gz1) = 24ae75ad1b44f17759c476575b4bfcf2
MD5 (file.gz2) = 640ee71b0db468212cfc8e5fe3be2884
MD5 (file.gz3) = dd18f21b48b516bd0b6d53dc404d6fcb

To verify your backups are fine, uncompress the first file from the split. You will get an error from gzip saying that the file is incomplete, but it will uncompress what it can. Use that for the checksum, and you will find all backups are the same. (Be sure you use gzip -d -c < file so it doesn't try and delete the file to replace it with the uncompressed one)

Another way to verify is to do dd if=file of=file.csum skip=1 bs=8

This may take awhile to do, since it is reading in 8 byte blocks, but it skips the first 8 bytes, wheich contain the timestamp. (The timestamp is in fact the second 4 bytes. The first 4 is probably some magic value for gzip.. Not sure for sure) Also if you do just gzip or gzip -9 or gzip -1, it will differ. I believe this difference would also be in the first 8 bytes.

[This message has been edited by Tiger (edited 08-28-2000).]

Posted by: gelee

Of course! The old timestamp in the second four bytes trick! I knew that! http://www.avsforum.com/ubbtivo/wink.gif
The gunzip process seems to be taking forever so I think what I'll do is restore everything to my new B drive and use the "runideturbo=false" trick to test it.
Thanks for the help. http://www.avsforum.com/ubbtivo/smile.gif