Document:

<PAGE>

[***] INDICATES THAT TEXT HAS BEEN OMITTED WHICH IS THE SUBJECT OF A
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                                                                   EXHIBIT 10.20

                                  ADDENDUM TWO
                                     TO THE
                       LICENSE AND DISTRIBUTION AGREEMENT
                                     BETWEEN
                               DELL PRODUCTS L.P.
                                       AND
                             COMMVAULT SYSTEMS, INC.

This Addendum Two ("Addendum Two") to the Software License Agreement dated
December 17, 2003 (the "Agreement"), is entered into by and between DELL
PRODUCTS L.P. (HEREINAFTER "DELL") WITH ITS PRINCIPAL PLACE OF BUSINESS AT ONE
DELL WAY, ROUND ROCK, TEXAS 78682, AND COMMVAULT SYSTEMS, INC., (HEREINAFTER
"COMMVAULT") A DELAWARE CORPORATION HAVING A PRINCIPAL PLACE OF BUSINESS AT 2
CRESCENT PLACE, OCEANPORT, NEW JERSEY 07757 AND IS EFFECTIVE AS OF THE 30TH DAY
OF OCTOBER, 2004 (THE "EFFECTIVE DATE").

                                    RECITALS

WHEREAS, COMMVAULT and Dell entered into the Agreement through which COMMVAULT
granted Dell various rights to distribute certain COMMVAULT software products;

WHEREAS, the parties now desire to amend the Agreement.

NOW THEREFORE, in consideration of the mutual covenants and promises set forth
herein and for other good and valuable consideration, the receipt of which both
parties hereby acknowledge, Dell and COMMVAULT agree as follows:

Any capitalized terms defined in this Addendum are specific to this Addendum
only, and do not modify or change the meaning set forth in the Agreement. Unless
expressly defined in this Addendum, the capitalized terms in this Addendum are
as defined in the Agreement. The Agreement shall remain in full force and effect
except as supplemented and amended herein.

      1.    The Pricing Supplement to the Agreement is hereby replaced in its
            entirety with the attached Exhibit B to this Addendum.

      2.    [***]. Dell shall coordinate with CommVault support to ensure that
            customers who receive support and product updates are eligible for
            such services. CommVault's MSRP for annual maintenance contracts is
            [***] of product list price. Dell's cost to CommVault is [***] of
            software cost and reflected in the Exhibit B to this Addendum.

No other changes or modification are intended by this Addendum. All other terms
and conditions of the Agreement are in effect.

IN WITNESS WHEREOF, the parties have executed this Addendum by their duly
authorized representatives as of the date first set forth above.

<PAGE>

COMMVAULT SYSTEMS INC.                    DELL PRODUCTS L.P.

By:    /s/ David R. West                  By:    /s/ Joseph Kanicki
   -----------------------------------       -----------------------------------
Name:  David R. West                      Name:  Joe Kanicki
     ---------------------------------         ---------------------------------
Title: VP Business Development            Title: Sr. Manager - WWSP
      --------------------------------          --------------------------------
Date:  11/15/2004                         Date:  11/22/2004
     ---------------------------------         ---------------------------------

<PAGE>

                                    EXHIBIT B
                               PRICING SUPPLEMENT
                        TO THE SOFTWARE LICENSE AGREEMENT
                           BETWEEN DELL PRODUCTS L.P.
                           AND COMMVAULT SYSTEMS INC.

<TABLE>
<CAPTION>
                                                                                                  DELL SW              1 YR. UPDATE
SKU           SKU DESCRIPTION       [***]        [***]           MSRP              DISCOUNT         COST                PROTECTION
------        ---------------       -----        -----        -----------          --------     ------------           -------------
<S>           <C>                   <C>          <C>          <C>                  <C>          <C>                    <C>
                                                 [***]
SKU1          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU2          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU3          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU4          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU5          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU6          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU7          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU8          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU9          [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU10         [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU11         [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU12         [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU13         [***]                              [***]        $     [***]           [***]        $    [***]             $     [***]
SKU14         [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU15         [***]                 [***]        [***]        $     [***]           [***]        $    [***]             $     [***]
SKU16         [***]                                           $     [***]           [***]        $    [***]             $     [***]
SKU17         [***]                                           $     [***]           [***]        $    [***]             $     [***]
SKU18         [***]                                           $     [***]           [***]        $    [***]             $     [***]
SKU19         [***]                                           $     [***]           [***]        $    [***]             $     [***]
</TABLE><PAGE>

[***] INDICATES THAT TEXT HAS BEEN OMITTED WHICH IS THE SUBJECT OF A
CONFIDENTIAL TREATMENT REQUEST. THIS TEXT HAS BEEN FILED SEPARATELY WITH THE
SEC.

                                                                   EXHIBIT 10.21

                                 ADDENDUM THREE
                                     TO THE
                       LICENSE AND DISTRIBUTION AGREEMENT
                                     BETWEEN
                               DELL PRODUCTS L.P.
                                       AND
                             COMMVAULT SYSTEMS, INC.

This Addendum ("Addendum") to the Software License Agreement dated December 17,
2003 (the "Agreement"), is entered into by and between Dell Products L.P.
(hereinafter "Dell") with its principal place of business at One Dell Way, Round
Rock, Texas 78682, and CommVault Systems, Inc., (hereinafter "Commvault" or
"Supplier"), a Delaware corporation having a principal place of business at 2
Crescent Place, Oceanport, New Jersey 07757 (hereinafter "Licensor") and is
effective as of the 1st day of May, 2005 (the "Effective Date").

                                    RECITALS

WHEREAS, COMMVAULT and Dell entered into the Agreement through which COMMVAULT
granted Dell various rights to distribute certain COMMVAULT software products;

WHEREAS, the parties now desire to amend the Agreement to provide that COMMVAULT
shall, for an additional fee, take on Level 2 Support obligations for new and
existing customers commencing on the Effective Date.

NOW, THEREFORE, in consideration of the mutual covenants and promises set forth
herein and for other good and valuable consideration, the receipt of which both
parties hereby acknowledge, Dell and COMMVAULT agree as follows:

Any capitalized terms define din this Addendum are specific to this Addendum
only, and do not modify or change the meaning set forth in the Agreement. Unless
expressly defined in this Addendum, the capitalized terms in this Addendum are
as defined in the Agreement. The Agreement shall remain in full force and effect
except as supplemented and amended herein.

1. Section 4.0 of Schedule C, Enterprise Support shall be modified by replacing
the existing first sentence in such section so that it reads as follows:

      [***]

2. Section 2 of Addendum Two to the License and Distribution Agreement, dated as
of October 30, 2004, which sets forth the price that Dell pays CommVault for
annual maintenance contracts, shall be amended in its entirety so that it reads
as follows:

      [***]. Dell shall coordinate with CommVault support to ensure that
      customers who receive support and product updates are eligible for such
      services. CommVault's MSRP

                                     Page 1
<PAGE>

[***] INDICATES THAT TEXT HAS BEEN OMITTED WHICH IS THE SUBJECT OF A
CONFIDENTIAL TREATMENT REQUEST. THIS TEXT HAS BEEN FILED SEPARATELY WITH THE
SEC.

      for annual maintenance contracts is [***] of product list price. Dell's
      cost to CommVault is [***] of software cost and reflected in the Exhibit B
      to this Addendum.

3.    A new Section 3.5 shall be added to the Agreement that reads as follows:

      In addition to the report set forth in Section 3.4 above, Dell shall also
      submit [***] royalty reports to CommVault within [***] days after the end
      of each [***].

No other changes or modification are intended by this Addendum. All other terms
and conditions of the Agreement are in effect.

                                     Page 2
<PAGE>

IN WITNESS WHEREOF, the parties have executed this Addendum by their duly
authorized representatives as of the date first set forth above.

COMMVAULT SYSTEMS INC.                   DELL PRODUCTS L.P.

By:     /s/ David West                   By:     /s/ Joseph J. Kanicki
   -------------------------------          ----------------------------------
Name:   David West                       Name:   Joseph J. Kanicki
     -----------------------------            --------------------------------
Title:  VP Business Development          Title:  Senior Manager
      ----------------------------             -------------------------------
Date:   4-28-2005                        Date:   4-28-05
     -----------------------------            --------------------------------

                                     Page 3
<PAGE>

                                    EXHIBIT B
                               PRICING SUPPLEMENT
                        TO THE SOFTWARE LICENSE AGREEMENT
                           BETWEEN DELL PRODUCTS L.P.
                           AND COMMVAULT SYSTEMS INC.

<TABLE>
<CAPTION>
                                                                                          1 YR.
                                                                SW           DELL SW      UPDATE        TOTAL
DELL SKU     SKU DESCRIPTION           [***]     [***]         MSRP           COST      PROTECTION       COGS
--------     ---------------           -----     -----        ------        --------    ----------      ------
<S>          <C>                       <C>       <C>          <C>           <C>         <C>             <C>
SKU 1        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 2        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 3        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 4        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 5        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 6        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 7        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 8        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 9        [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 10       [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 11       [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 12       [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
SKU 13       [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
             [***]
SKU 14       [***]                     [***]     [***]        $  [***]        [***]        [***]         [***]
             [***]
SKU 15       [***]                                            $  [***]        [***]        [***]         [***]
SKU 16       [***]                                            $  [***]        [***]        [***]         [***]
</TABLE>

                                     Page 4

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